Tuesday, February 23, 2010

Meta rears its head again, thanks to recent discussion on the thread that refuses to die. Short form, one commenter asked about the implications of space warps, and another pointed out that as magitech they are outside the scope of this blog.

I've never formally defined the scope here, but in a practical sense it is surely true. In fact, regular readers here know that I am downright stodgy.(I told Orville, and I told Wilbur, and now I'm telling you - that thing will never work.) My justification is that at least the doors to space, orbit lift and basic space operations, are mature technologies.

The space age is about as old, mature, and well established as commercial jets. And we have in fact achieved all the basic operational Cool Stuff, including building building a space station, keeping it in service over years, and test flights to the Moon, even if we did it in a contrary order.

What keeps us from doing more of it is not technology but how expensive it has turned out to be. We've had that discussion before, and will no doubt have it again.

There are other reasons for my bias toward Realistic [TM] technology. One of them - the underlying one, in fact - is aesthetics, which I will get to in another post. But another reason is that I like to play with numbers.

Technologies 'indistinguishable from magic' are not very easy to quantify, or if you do, the numbers are equivalent to spell points. Magical and technomagic systems can and should mesh together internally, but their rules are as arbitrary as the rules of football. Which is why those arguments about Trek versus Star Wars are like arguments about why World Cup teams never go for a touchdown.

Torch drives are another matter. I have always been partial to torchships, from the evocative name to the plausible implied physics. A torch is conceptually a rocket engine, just using some high energy reaction such as fusion. Yes, controlled fusion has been 20 years away for 60 years, but it is hardly magical to suppose that by 2100 or so, after 150 years of trying, we might get the hang of it.

Let us say that our ship has a mass of 1000 tons, and a modest exhaust velocity of 300 km/s, a mere 0.001 c. In the name of further modesty we will set our acceleration at less than one third g, 3 meters per second squared. This ship is very much on the low end of torchships; Father Heinlein would hardly recognize her. If our torch has VASIMR style capability, we can trade specific impulse for thrust; dial exhaust velocity down to 50 km/s to develop 1.5 g, enough to lift from Earth. (But see below.)

Setting surface lift aside, let's look at travel performance from low Earth orbit. We will reach escape velocity about 15 minutes after lighting up, a good Oberth boot, but with this ship it hardly matters. Suppose half our departure mass is propellant, giving us about 180 km/s of delta v in the tanks. Since we must make departure and arrival burns, our transfer speed (relative to Earth) is about 90 km/s.

Holding acceleration to 0.3 g as we burn off mass, we'll reach 90 km/s in less than eight hours, at about three times lunar distance from Earth. (If we were going to the Moon, we'd need to swing around three hours out for our deceleration burn.)

For any deep space mission we'll end up coasting most of the way, and 90 km/s is some pretty fast coasting. Even this low end torchship lets you take retrograde hyperboloid orbits, which is Isaac Newton's way of saying you can pretty much point & scoot.

You will truck along at 1 AU every three weeks, reaching Mars in little more than a week at opposition, though up to two months if you travel off season. The near side of the asteroid belt is a month from Earth, Jupiter three months in season. For Saturn and beyond things do get prolonged.

Still, 90 days for the inner system out to Jupiter is nothing shabby, and there's no apparent High Magic involved.

How much power output does out modest torch drive have? The answer, skipping simple but tedious math, is not quite half a terawatt, 450 GW. This baby has 600 million horses under the hood. That's effective thrust power. Hotel power for the hab is extra, and so is getting rid of the waste heat.

Putting it another way, this drive puts out a tenth of a kiloton per second, plus losses.

And yes, Orion is the one currently semi plausible drive that can deliver this level of power and performance. I say semi plausible because, broadly political issues aside, I suspect that Orion enthusiasts gloss over the engineering details of deliberately nuking yourself thousands of times. Like banging your head on the table, it only feels good when you stop.

The good news, for practical, Reasonably Fast space travel, is that we don't need those near-terawatt burns. They don't really save much time on interplanetary missions - we could reduce drive power by 90 percent, and acceleration to 30 milligees - the acceleration of a freight train - and only add three days to travel time.

(Our reduced 'sub-torch' fusion drive is still putting out a non trivial 45 gigawatts of thrust power, which happens to be close to the effective thrust power of the Saturn V first stage.)

If you want Really Fast space travel, however, you will need more than this. Go back to our torchship and increase her drive exhaust velocity by tenfold, to 3000 km/s, and mission delta v to 1800 km/s, while keeping the same comfortable 0.3 g acceleration.

A classic brachistochrone orbit, under power using our full delta v, takes a week and carries us 270 million km, 1.8 AU. Add a week of coasting in the middle and you're at Jupiter. Saturn is about three weeks' travel, and you can reach distant Eris in 6 months.

Drive power output of our upgraded torchship is now 4.5 TW, about a third the current power output of the human race. Which in itself is no argument against it. Controlling the reaction and getting rid of the waste heat are more immediate concerns.

As for a true, Heinleinian torchship? Heinlein's torch is a mass conversion torch. He sensibly avoids any details of the physics, but apparently the backwash is a mix of radiation, AKA photon drive, and neutrons, probably relativistic.

Torchship Lewis and Clark, pictured above, is about 60 meters in diameter, masses in on the order of 50,000 tons, and in Time for the Stars she begins her relativistic interstellar mission by launching from the Pacific Ocean at 3 g. I don't know how to adjust the rocket equations for relativity, but the naive, relativity-ignoring calculation gives a power output of 225 petawatts, AKA 225,000 TW, AKA 53 megatons per second.

Do not try this trick at your homeworld. I don't know whether Heinlein never checked this calculation, did it and ignored the results, or did it and decided that a few dozen gigatons - 12 torchships are launched, one after another - was no big deal since it was off in the middle of the Pacific somewhere.

Maybe he only did the calculation later, because in other stories the torchships sensibly remain in orbit (served by NERVA style nuke thermal shuttles).

You don't need full Heinleinian torchships even for fast interplanetary travel, but there is another side to this whole discussion. Over a couple of hundred years, the era I've been calling the midfuture, would we really expect the propulsion technology to remain essentially unchanged?

But directly in front of the gate hardly half a mile away was a great ship that he knew at once, the starship Asgard. He knew her history, Uncle Chet had served in her. A hundred years earlier she had been built out in space as a space-to-space rocket ship; she was then the Prince of Wales. Years passed, her tubes were ripped out and a mass-conversion torch was kindled in her; she became the Einstein. More years passed, for nearly twenty she swung empty around Luna, a lifeless, outmoded hulk. Now in place of the torch she had Horst-Conrad impellers that clutched at the fabric of space itself; thanks to them she was now able to touch Mother Terra. To commemorate her rebirth she had been dubbed Asgard, heavenly home of the gods.

190 comments:

I am going to throw my support behind scientifically plausible magitech. These are tricks like Krasnikov tubes, Alcubierre/Van-der-Broeck warp drives, and traversable wormholes. General relativity allows a number of solutions of getting from here to there faster than a photon chugging along through flat space-time, and some of these solutions can even be accomplished with less than Jupiter masses.

The fun thing about scientifically plausible magitech is that it leads you in all sorts of unexpected directions. You get interesting restrictions on what is possible and often your setting takes delightful unexpected twists when you consider the implications. Sometimes, you end up having to ditch cool ideas - much like ditching space fighters. For example, why take a rocket ship through a wormhole to Zeta Reticuli rather than getting on a tram through the Spokane wormhole gate directly to Port Kato, Zeta Reticuli Prime?

And as for torch ships - a bigger problem than creating the hellish inferno of nuclear fire needed to propel your spacecraft is keeping your spacecraft from evaporating under the intense x-ray, gamma-ray, and neutron irradiation. My best guess for accomplishing this is keeping the torch flame outside the spacecraft, and couple the plasma to the spacecraft using magnetic fields to give you thrust. The latter trick probably means several conductive - or, more likely, superconductive - loops of cable surrounding the torch flame but at a good healthy radius. To protect your field generating cable, you will need to shield them, probably with blade-like tungsten structures edge-on to the torch. The large surface area of the blades gives you a lot of radiator surface while only intercepting a small amount of radiation (only what is needed to shield the cables), tungsten does a good job of scattering neutrons away without heating up much, at shallow angles of incidence x-rays (but not neutrons or gammas) are reflected away, and tungsten can heat up to yellow-white hot without evaporating too much. An unusual consequence of the latter, and the relatively small emissions from your optically thin thrust plasma, is that visually the tungsten shields will be the brightest part of your spacecraft when under thrust - the intensity will be the same as that of an incandescent filament from a light bulb, but you will have a lot more area to radiate from.

These resulting torch craft don't end up looking much like conventional rockets. You get loopy filigree and from the support and field cables, and graceful glowing sheets from the heat shields, enclosing a volume that is probably much larger than the passenger/payload section.

"Which is why those arguments about Trek versus Star Wars are like arguments about why World Cup teams never go for a touchdown."

Yes. But don't try to explain this to the people having the argument.

It might turn out that fusion torch-drives are plausible, while fusion power is expensive and difficult. If torchdrives use a series of micropulsed fusion events, shaped and directed by external magnetic fields into a rocket-plume, that technology might not translate well into commercial power generation. You could have torchships scooting around while everyone on Earth uses solar panels and wind/tidal mixes.

Performance in spacecraft, much like cars and aircraft, is directly affected by mass. Without going into numbers (mostly because I'm feeling lazy tonight), I would suggest that very high performance spacecraft are possible once the technology of energy beaming becomes mature.

Earth to Orbit is already conceptually possible, with Leik Myrabo actually having launched proof of principle laser "rockets". Since these devices use the atmosphere as remass for the initial launch and to the edge of space, they have effectively infinite ISP, and proposed space launchers use a tank of L2 for that final kick into space, resulting in ISP's in the mere 1000 second range in vacuum. Check out this Japanese video: http://www.youtube.com/watch?v=-Nm16wp0kMs

Further afield, a VASMIR engine powered by a 2 Mw nuclear reactor can get to Mars in 39 days; without the mass of the reactor and shielding, the ships performance would be considerably enhanced. A powerful laser or maser system at the Earth-Sun L1 point beaming 2 or more Mw at a suitable receiver on the ship would make this possible. Since removing the mass of the reactor eliminates the need for extra fuel, remass and a large portion of the cooling system would not be required, the ships performance would be even better (or far less power is needed to reach Mars in 39 days).

Luke mentioned "scientifically plausible magitech". There are experiments on the Mach effect which seem to produce reactionless thrust: http://nextbigfuture.com/2010/02/mach-effect-propulsion-research-update.html

I will leave that one for readers to digest. If this is true, there are probably all kinds of other second and third order effects which will be entirely unanticipated due to our new understanding of the universe.

"a VASMIR engine powered by a 2 Mw nuclear reactor can get to Mars in 39 days; without the mass of the reactor and shielding, the ships performance would be considerably enhanced. A powerful laser or maser system at the Earth-Sun L1 point beaming 2 or more Mw at a suitable receiver on the ship would make this possible." - Thucydides

I've made a similar suggestion of using a solar-powered laser to externally transmit required energy to solar-electric drives out past the orbit of Mars in an earlier blog entry. As is then, a question will ultimately pop up for such a power transmission system: what's to keep those operating the laser as a weapon? The thing must be generating a high amount of power within the beam itself, potentially enough to cause serious damage if the "turret" for lack of a better word isn't aiming at the correct part of the spacecraft.

As for the blog entry in question, from the numbers Rick had provided, I was under the impression that an acceleration of 1 G and a high enough DeltaV budget to cover the one-way journey for both the acceleration AND the deceleration after the midpoint (Or just the coast in the middle for rocket drives that aren't that powerful) at least were what constitutes for a "proper" Torch Drive. From what I can figure, I guess fractions of G would be acceptable accelerations though I would imagine that 1 G would be preferred for the onboard crew to not suffer the detrimental effects of low gravity onto one's body once they reach their destination.

And considering Joh's Law and the assumption that any Torch Drive or Torch-Rated Rocket engine is going to be extremely complex to build and maintain, such timescales would be out of the reach for much of interplanetary commerce and shipping for some time to come. Chances are that military armed forces (and some government agencies that have the budget and need) will be the chief, if not sole users of Torch Drives. Everyone else from high speed passenger spacecraft to freight-cargo will have to deal with Hohmann Orbits and launch windows.

Though then again, interplanetary passenger lines would have to be more faithful to the departure time rather then just rely upon the "adverse weather conditions" causing a delay.

Power beaming stations might well be dual purpose, the space age equivalent of the military frontier posts of the American west.

The military purpose would be to protect Earth from infalling asteroids or whatever military threat develops in deep space, but they pay for themselves by beaming power to cooperative targets like friendly shipping or energy receivers mounted on NEOs. Unless there is a red alert, shipping takes priority and even if the beam is interrupted, the ships continue to coast on predictable orbits and can be picked up after the interruption is resolved (repairs made, asteroid vapourized etc.)

Life in Fort Heinlein revolves around maintaining the solar energy arrays and maintaining the tracking systems, and life will be pretty tedious. Daily routine includes system checks and battle drills, and screw-ups get to go out and polish the mirrors under the first sergeant's unforgiving gaze. A secondary economy of service providers ( saloons and whorehouses) will grow around the "fort" to service the crew, and other business might set up shop as well, everything from contractor repair depots to futures traders monitoring ship traffic and energy consumption.

Lightweight ships tapping into this system have torch like performance, economy traffic might go by cycler (although the "taxis" might need torch like performance to match the cycler or slow down to orbital velocity after dropping off) and bulk traffic will still go by low cost transfer orbits.

This mid-range future is starting to look more like aircraft control all the time.

There are airports - Cyclers, stations, habitats, and other structures that act as waystations. These structures have restricted approaches and 'airspace'.

There are flight plans - Transfer-orbits, laser-propulsion paths, and for the high-performance-craft those retrograde hyperboloid orbits.

There are security threats - What happens when some narcissistic whiner decides to fly a shuttle into a local government office?

There are accidents - Like modern aircraft accidents, most spacecraft accidents will probably take place close to a station. And the authorities won't be able to do much until it's over. The good news is that, like aircraft accidents, spacecraft accidents are likely to be rare.

There is no silence - There might be lag but you will get your email, text messages, tweets, and YouTube channel updates. In space, everyone can hear you drunken-post.

Even 'subtorch' high end drives are likely to have an open latticework structure to support a magnetic containment 'nozzle.' I like to call this structure a lantern, because it would glow very bright when the drive is operating.

Love the Sci-Faiku!

The only real seasons in space are travel seasons, as I call the cycle of launch windows.

Torches might indeed be feasible without fusion power generation being practical - a torch, really, is a semicontrolled explosion (or series of pulses), that needs to recover only enough power to maintain the cycle.

Energy beaming could turn out to be very attractive, especially between regular travel destinations, allowing more use of the infrastructure.

But if we have some beaucoup fast form of travel, such as a torch, I think it will be used for for passenger travel. Beyond the convenience saving there are operational savings in simpler life support, reduced shielding requirement, etc.

One of the biggest differences between real space and rocketpunk era space is comms. There is physical isolation but rarely communications isolation in the Vastness of Space.

At one point of the story (because, alas, they are still science-fiction), someone always say “… and it becomes a terrific weapon were you to hover it over a city”. From my childhood, I remember a “Space 1999” episode about a Mr Keiller who sent a space probe which vitrified his protégé’s native city; back then, it didn’t make sense to me.

Just because the exhaust is from a fusion torch does not mean it has anything “more” than a conventionnal honest-to-goodness, run-of-the-mill, perfectly normal, mundane, ordinary, ho-hum boring chemical rocket.

I mean, whether it is nuclear powered or not, a given spaceship will always need the same energy to go up, at least in orbit if not further.

Last time I checked, Cape Kenn^h^h^h^hCanaveral wasn’t vitrified when Neil & Co. blasted-off for the moon on the biggest thing ever to blast-off from Earth…

So why a fusion rocket would vitrify cities when a Saturn V rocket would not?

The smaller torchdrives, like the 45 gw example Rick discusses in the main post, wouldn't slag a city. The problem is that space enthusiasts and SF writers have been tossing around huge numbers for so long that readers think those numbers are normal. Some hard SF writer or engineering group talks about a hypothetical .3g drive, someone else points out that said drive spits out 4.5 terawatts of exhaust, and everyone assumes that 4.5 tw and .3g are reasonable numbers.

But as Rick points out (And I've seen other commentators discuss this) you don't need all that sturm and und drang to get around the solar system. A steady thrust measured in thousandths of g will get you where you want to be in a humanly useful timeframe.

Also, yes you can kill a city by parking a spacecraft over it. But it would be quicker and less expensive to just nuke the place. Do modern militaries try to kill each other with exhaust fumes from tanks?

Actually, given the great time lags in communications (and probable bandwidth issues), I would not expect interplanetary Tweets, phone calls or even emails.

Point to point communications would most likely resemble texting (including the incomprehensible abbreviations), but you would have anywhere from 1.4 seconds delay to the moon to many hours to Uranus.

This means another possible driver for high performance spacecraft would be mail delivery. (Consider a small car delivering 100 DVDs for Netflixx probably is carrying more information than you can access through your home internet connection in a day.) Contracts, magazines, movies, personal messages and anything else which needs more detail than a text message will all be loaded aboard mail servers on fast packets, and blasted to their destinations via the fastest means possible.

This of course leads to interesting scenarios where protecting and intercepting mail becomes important for intelligence agencies, business and criminals. Mail delivery will involve high levels of security and screening of the mail delivery personnel. I doubt anyone will hijack a mail packet in flight, but having a covert operative on board to hack the mail server and download the interesting information is a very real possibility.

Emdx: A 1 TW torchship hovering over a city will be putting out the energy of a 1 kiloton blast every four seconds, of a Hiroshima bomb every minute. I imagine this would not be good for property values. However, the exhaust will be quite diffuse. A quick back-of-the-envelope calculation assuming a 1 TW power output and a 100 ton craft hovering in 1 gravity gives it putting out about half a kilogram of exhaust per second, as a low density jet. This will not travel very far through the air, imparting most of its energy into a fireball at the exhaust nozzle of the spacecraft. It will be the torch-ship that will be absorbing the full brunt of the 1 Hiroshima per minute fireball, while the city below it will only need to worry about the radiated heat. I even have my doubts that the drive would work at all in an atmosphere - air might well flood the reaction chamber, interfering with the ability of the plasma to reflect off the magnetic nozzle.

Rick: I'll haul out my PhD in physics and the work I've done in general relativity to mention that wormholes, warp drives, and Krasnikov tubes are viable solutions of Einstein's equations of general relativity. They require some rather odd conditions, namely regions of space-time with negative energy densities. We know this is not unphysical, since there are odd cases we know or strongly suspect exist with negative energy densities (black hole event horizons, the Casimir effect between nearby conducting surfaces). The fun stuff tends to require an awful lot of negative energy, but the amount needed tends to keep getting smaller with more research.

A few highlights of the various space-warping methods:

Wormholes are shortcuts through space-time. One end of a wormhole connects on another end, and going through takes you somewhere else in space and time. Wormholes are two way - you can go back again, and going through a wormhole may (or may not) involve strong tides but is otherwise just like traveling through any other region of space (none of this shimmery barrier like you see in StarGate). It is strongly suspected, but not yet proven, that a wormhole cannot take you farther back in time than it would take for a light signal to propagate from where you are going to where you left - in otherwords, wormholes can be used for FTL but not time travel (in relativity jargon, they only connect space-like intervals). Trying to move a wormhole around so as to make a time machine is thought to result in the destruction of the wormhole (or possibly just large forces that prevent the wormhole from entering into configurations that let you travel into your own past). All conserved quantities are conserved locally at wormholes - if a wormhole end has a given mass, pushing something with extra mass through the wormhole from that end will add its mass to the wormhole end, while if something comes out of that end, its mass will be subtracted from that end of the wormhole. The same goes for electric charge and (in a vector sense) momentum. If wormholes cannot have negative mass, this limits the amount of stuff you can send one-way through a wormhole before needing to send more mass back the other way. Many Sci Fi authors posit wormholes orbiting around stars in the vacuum of space, but there is no real reason I can think of not to have them located some place more convenient, such as in the aforementioned Spokane, WA. You would probably want to put them in an airlock to keep all the air from whooshing through from high pressure to low, and if you have more than one wormhole you will need to be careful that that there are no round trips you can take that bring you back into your own past (because if there was, some wormhole leg of that trip would collapse to prevent this).

Warp drives let you take a spacecraft and warp space-time around it so that a bubble of space-time around the spacecraft surfs through space-time at an apparent superluminal rate. The spacecraft, however, is at rest inside its bubble and is not actually moving. The most plausible form yet devised is the Alcubierre/Van-der-Broek geometry, which pinches the spacecraft off into a pocket universe connected by a microscopic wormhole to our universe through a region smaller in volume than a proton. Then you warp the microscopic wormhole end rather than the huge volume of the entire spacecraft. Clearly, the spacecraft would be blind while warping. There are unresolved issues with a warp drive - when moving at super-luminal speeds you get a singularity "bow shock wave" at the front of the bubble, which may not be physical (we are not sure yet). Also, when going super-luminal, the spacecraft is causally disconnected from the rest of the universe, so it could not maneuver while warping, only travel on a pre-planned course. These last two limitations go away if you only use the warp drive for sub-luminal journeys (making a warp drive a sort of reactionless drive). The conservation laws still hold - if you warp close to a planet, the planet's gravity will pull on the warping craft and change its velocity, building up momentum toward the planet.

Krasnikov tubes are not well researched yet, but they seem to work. You prepare a path through space-time along which material objects can move back and forth at apparent super-luminal speeds. This is sort of like an interstellar rail line.

Note that none of these tricks allow local faster than light motion through space-time - you only seem to move faster than light to distant observers.

Well, I should point out that it is the height of hubris to believe that we know all the secrets of the universe. The known universe is larger than it is old and thus at some point was expanding FTL and it seems to be accelerating. One explanation is that space itself (not matter, but space) is welling up from vacuum energy. Thus the matter of the universe isn't moving FTL, the space in which the matter exists is moving FTL. It is simple to make the assumption that if space can be created, it can also be collapsed, thus bringing things closer together and allowing sublight transit to distant places. The fact that we haven't witnessed any space shrinkage (especially given the cold of space) doesn't mean it can't happen.

Thucydides - "Actually, given the great time lags in communications (and probable bandwidth issues), I would not expect interplanetary Tweets, phone calls or even emails.

...This means another possible driver for high performance spacecraft would be mail delivery."

This sounds very implausible to me.

The light speed delays mean that all interplanetary communication will resemble emails with attachments more than phone calls.

The immense power required for torchship performance would make interplanetary communication lasers much more economical for sending even terabytes of data, much less an email greeting with a 1 MB photo.

Using torch drives to fry cities, like dropping boulders on them, is one of those Rube Goldberg things that fascinates the SF imagination. If you are out to destroy human beings and their works in large numbers, just nuke 'em.

That said, even a 'true' torch drive might have 100 times the power output of the Saturn V, and would make a considerable mess of the launch area.

On the third hand, I agree with Luke that these drives might not work in the atmosphere anyway. Trying to run a torch immersed in cold, dense gas is a bit like trying to run a jet engine underwater.

Winch, if you're lurking, you should swipe Luke's whole mini-lecture for Atomic Rockets. From a writer's point of view one key question is what sort of FTL-ish journeys are potentially possible, and which ones are not. And what exactly happens if you attempt one of the 'illegal' ones.

Space mail should have no real problem with lag - we don't usually answer email in real time. People comment on this blog from Europe while I'm sleeping; it would make no difference if their comments were transmitted from Saturn space.

Where I see a place for fast transport is package express, when it absolutely, positively has to reach Mars next month. I'd expect passenger liners to have an express room, or clamps for express pods. Express capsules could also be given a laser boot, or similar means.

The range we're getting on long range communications is due to directional antennae. If you're broadcasting omnidirectionally, the signal drops fast. So you could get radio transmissions, but there would like be just the one line. That one line would also be blocked by planets and other stuff on occasion. That one line would be in high demand and thus not readily available. You might spend thousands on a couple minute recording that gets compressed and burst fed to a destination. Real messages, sent parcel post, would probably get stored digitally and then transmitted once you arrived at the destination orbit.

To expand on Citizen Joe's point, would you like to receive "Male enhancement" emails with the olympusmon.mars address? How about "Dear Sir, I am the last surviving member of the Europan Resistance front and need your help to transfer $10 billion solars from the Bank of Callisto...."

On a more serious note, the high bandwidth links would probably be reserved for ship traffic, government and military communications and corporate communications (for companies with the financial clout to get in line for email). Certainly the Uranus Space Navy would not want the high bandwidth links clogged during their showdown with the Imperial Jovian Navy, nor would they want to risk malware or botnet attacks coming through those links; which suggests interplanetary comms would be tightly controlled and subscribers carefully vetted.

Here's a question, assuming you have mastered the technology to contain and sustain a fusion reaction, and that containment doesn't cost you more energy than you're getting out, how do you get electricity from that reaction? I'm thinking in regards to a spaceship with this.

The obvious (to me) way is to use a working fluid and a thermocouple to make use of heat differences to make electricity. If this is the case, you've got to have a lot of low temperature radiators to get more efficiency. It also seems like a generation or two behind a what a space-ship power generator should be.

Can you run some of the output plasma (the ones that aren't screaming out the tail of your ship) through a magnetohydrodynamic dynamo?

If this is the case, might a spacecraft have multiple stages of reactor output? Stage 1 is a low power, sustained reaction that has most of its products used for electricity generation. Stage 2 is a high power reaction where most of the products are directed rearward.

My knowledge of fusion is limited to what I learned from college physics, so I know some basic theory, but that's about it.

Micheal: Extracting energy directly from the hot plasma is likely to be the most efficient and least complicated method of getting electricity out. This would be the case of the magnetohydrodynamic generator you mentioned. It also lets you dump most of your entropy out with the plasma, so (unlike the case with heat engines, thermocouples, and working fluids) you will not need an even larger radiator.

"The range we're getting on long range communications is due to directional antennae. If you're broadcasting omnidirectionally, the signal drops fast. So you could get radio transmissions, but there would like be just the one line. That one line would also be blocked by planets and other stuff on occasion. That one line would be in high demand and thus not readily available."

This is really just an engineering issue, and not that hard to solve. Signal multiplexing, multiple frequencies, different modulations, and even using lasers (which can pack in massive amounts of data) could (and would) increase the amount of bandwidth available in space.There's really nothing in the laws of physics that says you can't beam 1,000+ Tb/sec of data from earth to mars. It'll just take a few hours to get there. That itself could create some interesting cultural dynamics. No sane 'belter would ever play day trader on Venus' stock markets, for example.

That said, I agree with the points made above that you would see a massive amount of data security being used in space. The military forces would be the most paranoid, probably using laser transmissions most of the time to prevent eavesdropping. I like the idea of using a cost formula for data transmission that discourages spamming. I could see something like that being used to govern the use of non-corporate civilian data traffic.

Most of the energy created by fusion is high energy neutrons or gamma rays. Both would be captured by water, converting them to heat. The water then boils and turns a turbine.

Helium3 fusion throws protons. Being charged, it is relatively easy to capture that energy directly.

Note that where you need the power of a fusion plant is in the thruster. In that case, heat is actually a good energy source. The secondary power requirements can come from a stirling engine that soaks some of the waste heat to turn a flywheel and thus generate power.

Citizen Joe: I can't think of any fusion that releases gamma rays except for the fusion of protons, which is likely to be unusable for effective power or propulsion because it relies on the weak nuclear force. Annihilation, conversion, baryon decay, and other exotic methods of energy conversion that could potentially power a torch-craft may well release gamma rays, though, particularly if heavy nuclei are involved.

Neutrons are emitted in annoying quantities from most fusion reactions, as well as from most exotic energy generation methods I can think of that involve nuclei heavier than basic hydrogen. For fusion that produces lots of neutrons, you will want to intercept as few of them as possible, letting as many as you can escape into space in order to reduce your thermal load (and prevent other problems fast neutrons cause, such as embrittlement and amoprhization of your structural materials). While those parts of the spacecraft exposed to the neutron flux from the reaction will get very hot, trying to use that heat will just make your heat rejection problems that much harder. That is why I suggested simply tapping a portion of the plasma flow for power - this is very efficient and the waste heat mostly stays in the plasma.

There are also methods of directly using thermal x-rays or gamma rays for electricity. For example, you can use the photo-electric effect to charge up conductors exposed to the thermal x-ray flux. However, this does add heat from the x-rays you capture.

Just a note - if your torch requires 1 TW for thrust, this is the power that remains in the charged particles that you can divert into the plasma jet. If you are producing 1 TW of thrust from D-T fusion, in which 80% of the energy comes out as neutrons, the full fusion reaction will be producing 5 TW total.

I recently read about a fission drive concept known as a "fission sail" in which a solar-like 'sail' is coated with uranium and 'sprayed' with a particle beam composed of antimatter...resulting in a fission reaction and a huge amount of thrust. The article didn't specify the spacecraft configuration; however, some billboard-like panels coated with uranium, clustered around the beam emmitter/s at one the end of the ship (with the crew/mission modules at the other), would seem to be a resonable design. At least to me.

Most "exotic" aneutronic fusion reactions use 3He or proton-Boron reactions and release energy in the form of charged alpha particles, which can be directed by magnetic fields and trapped to extract high quality electrical current. Dispensing with Carnot cycle energy generation reduces ship mass considerably as well (no boilers, turbines, small radiators etc.)

Migma, Dense focus fusion, IEC Polywell machines and magnetized target fusion devices are theoretically capable of these sorts of reactions (and progress is happening far faster than the big government programs like tokomacs or laser IC), but of course we haven't seen sustained fusion from any device at all.

Perhaps the strangest non magitech torch drive I have seen is the "Medusa" version of ORION, where the massive pusher plate is replaced by something resembling a solar sail, and the massive shock absorbers are replaced by tension members (cables), causing the ship to be towed along behind the drive...With an ISP in the 50-100,000 second range, someone will want to try this.

Some of this post confuses me. Note that I do not have a college degree. However, it was written:

"Our reduced 'sub-torch' fusion drive is still putting out a non trivial 45 gigawatts of thrust power, which happens to be close to the effective thrust power of the Saturn V first stage.)

If you want Really Fast space travel, however, you will need more than this. Go back to our torchship and increase her drive exhaust velocity by tenfold, to 3000 km/s, and mission delta v to 1800 km/s, while keeping the same comfortable 0.3 g acceleration.

A classic brachistochrone orbit, under power using our full delta v, takes a week and carries us 270 million km, 1.8 AU. Add a week of coasting in the middle and you're at Jupiter. Saturn is about three weeks' travel, and you can reach distant Eris in 6 months.

Drive power output of our upgraded torchship is now 4.5 TW, about a third the current power output of the human race. Which in itself is no argument against it. Controlling the reaction and getting rid of the waste heat are more immediate concerns."

So, basically, you are saying that 20 times the power of the Saturn V first stage is impossible to do?

Ferrell, for your uranium fission sail, an antimatter beam would do the job of getting things cooking, but I think a much cheaper way to do the same thing would be to instead use a neutron beam from some outrageously radioactive emitter.

Your comment re sails, though, made me remember some rough back-of-the-envelope calculations I made a while ago for solar sails. In the inner system, a sail of sufficient size and good design would have a thrust level comparable to some of the subtorch drives mentioned earlier.I can imagine a society that has the engineering chops to build a functional torchdrive (of any appreciable thrust level) would also have the kind of materials science to make solar sails economically feasible. They'd be slower than a torch but still much faster than Hohmann transfers and less expensive per transit than either (assuming spherical-cow economics and engineering).

Scroll down on this page to see a table with the engine listing. VASMIR and DUMBO drives are listed. Toward the bottom you will see the fusion torch drive numbers.

The simpler, Non-mathematical answer:

Ion drives are good at "fuel conservation" but have low accelerations. These are good for long duration, long distance flights, where the time it takes to get there matters a lot less than getting there at all.

Chemical rockets, like the Saturn V stages, have staggeringly poor conservation and great acceleration: they are able to take off from Earth's gravity well, but it takes something the size of a Saturn rocket booster worth of propellant to do it.

Torch drives are good at both, allowing you to accelerate and decelerate over long distances using less reaction mass, and to do so at a good fraction of g allowing you too get there within a reasonable time frame.

To sum up: an ion or VASMIR drive can go to the end of the solar system on little reaction mass/propellant, but it will take years and decades. A Saturn rocket can put out several g's of acceleration, needed to get off Earth (have to accelerate up faster than the Earth accelerates you down) but go through fuel like crazy. Torch drives might not get you out of Earth's gravity well, after all 0.3g won't get you off the ground, but it's better than the 0.0001g of an ion drive. It might not have the fuel efficiency of a VASMIR, but it doesn't guzzle fuel like a 1950's Caddy with a hole in the fuel tank.

So obviously, neutrons are useless for providing thrust via the traditional magnetic nozzle that we use to direct fusion products. How much of that energy can be gained by heating water propellant? A cursory Google search doesn't tell me how many neutrons are absorbed in a given distance by water.

I'm trying to figure out if it is practical to use water propellant as an "afterburner" on a neutron rich fusion reaction. The idea being that most of the time charged products are used as propellant, while neutrons are allowed to escape, but if thrust was needed, you surround the reaction with neutron absorbing water, capture more energy, but lose a lot of exhaust velocity.

Water as a propellant isn't particularly good. However, it is readily available, even in space. So if you plan your trips by hopping from iceberg to iceberg, it doesn't matter that you're inefficient. Water also has a pleasant benefit of not wrecking the environment.

Another trick I saw recently was storing heat with molten salts. You can then pump the molten salt through water to create steam and thus electricity on demand.

But then how much more mass do you have to embark to have such systems? Aren't you going to quickly arrive at a point where storing all that water and/or steam turbines (in Citizen's power generation idea) will require more energy than you could retrieve? It seems that "wasting" all that energy is actually cheaper than adding the heavy systems needed to exploit it. Your mass budget is far more critical than your energy budget, and the balance between wasting energy and adding mass is very much skewed towards: go ahead and waste it.

Hello, I joyously discovered this site a few months ago and have expended a significant fraction of my scarce free time in keeping up with the posts and reading all of the archives (whew).

Since the subject of communication has come up in these comments, at some point I would love to see some discussion of what kind of communication technology an interplanetary civilization is likely to use for long-distance messaging. Radio? (Isn't that what we use now, for communication with interplanetary probes?) Maser? Laser?

Even the amazing nuts-and-boltsy Atomic Rockets site of Winchell Chung doesn't seem to address this question specifically, that I can find.

For example, my intuition tells me that any spacecraft outfitted with a laser capable of communication between, say, Neptune orbit and the Earth would also be able to use that laser as a weapon at shorter distances. If so, even a "peaceful unarmed merchantman" would have a potential short-range antiship/antimissile weapon. Interesting implications.

However, my intuition is frequently a lying bastard. Moreover, I am sure the potential power of such a laser would vary depending on the type of long-distance messaging it is used for -- i.e., full video/audio vs. "HAVE ARRIVED TRITON PORT STOP OFFLOADING CARGO STOP."

I would love to see some discussion by others who are more comfortable with the physics, engineering and mathematics involved.

Injecting water or other materials into the plasma stream of a fusion rocket would have the effect of lighting an afterburner on an F-16; gobs of thrust at the cost of huge amounts of remass.

This might be a common technique for military craft, a high ISP system for cruise and a high thrust, low ISP system for combat and emergency manoeuvres. How well these systems work will influence how ships are built (strongly built if they can achieve bursts of high "G", fragile if they only have to take a fraction of a "G")

As noted, so long as the material can be easily accessed, this is a very useful means of getting from point a to point b.

"Radio? (Isn't that what we use now, for communication with interplanetary probes?) Maser? Laser?" My guess would be, all of the above. As for lasers, terrestrial free-space laser communication systems get about 1Gb/sec for high-end systems. That's actually much lower than what is possible with optics, because of the problems inherent in trying to punch that laser through a messy atmosphere. In space, you could expect to get performance in excess of 1Tb/sec, YMMV.Radio is likely to be used extensively, to the point that the general space around planets and other points of interest could get pretty noisy. I'm not sure whether it's easier to collimate a laser beam or a radio signal over stupendous range. Whichever one wins would be the one you'd see more in interplanetary communications (someone with experience in both would need to clarify this).

@ Brian:With ion engines, chemical engines, and nuclear torches we're facing a classic Newton's Third Law problem. Somehow the exhaust needs to have sufficient momentum for the opposite reaction to give the ship a good acceleration. Chemical rockets solve the problem by expelling a ton of mass at a relatively low velocity. Ion drives expel a tiny amount of mass, so to get anywhere they get it moving FAST, but even at gigawatts of power they get a measly 0.0001g. Torch drives take a small-to-moderate amount of mass and use nuclear destruction to get it moving insanely fast. They're the only ones (insert disclaimer) with enough power per unit of reaction mass to get .3g constant acceleration conveniently. Even a perfect ion drive would need a phenomenal (read: impossible) amount of power input to match the performance of a nuclear explosion.

@Brian, Eric, on the ion drives: I think (and have read) the low thrust with electrostatic drives is not so much a problem of needed power (they've been talking giga-terawatts around here), but a problem of electrostatics. You need electric fields to accelerate the ions, but the ions generate electric fields of their own. This places a limit on how much current (~mass flow) you can put through an ion drive. If you put too much, the fields get saturated and can no longer accelerate more ions.

I think plasma thrusters like VASIMR have a better performance in terms of their maximum thrust, because they're not limited by electrostatic fields (though plasma does generate a magnetic field, which can lead to a thrust limit, though I'm not really sure of this).

@Stevo Darkly, about communication:Communication with electromagnetic radiation is subject to diffraction just like lasers. The diffraction equation is on Atomic Rockets, in the conventional weapons -> laser cannon section. Basically, the shorter wavelength you use, the less diffraction you get and the smaller angle you can focus the beam. Now, radio waves have much longer wavelength than light, so light from lasers could be used in a more efficient way for communication, as it can be concentrated better.

There are phased arrays for radio waves, and optical phased arrays are possible but cannot be built by existing technology. These would help to focus the transmission. I do not know any equations for phased arrays, though, so someone with more knowledge on these might want to enlighten us.

Since power and surface area aren't problems on the ground (and to a lesser extent, in orbit), communications would likely be predominantly TO ships while ship to shore (home base) comms would be limited to a confirmation of receipt. On Earth, we can afford to put up huge arrays to catch the smallest radio signal. Not so much in space. Likewise we can pump a lot of energy into the antenna to send back a longer message with a lot of bandwidth and strength even at stupendous ranges.

Now there are some tricks, like omnidirectional beacons and antenna that act as targets for the directional antennae. But you can only listen to data in the direction of the directional antenna. That might be limited to a single stream. Comm relays would likely have at least 4: Signal in, Signal out, Previous relay, Next Relay. By using multiple relays (at least 4 would get you around the sun) you wouldn't have black outs (except at ship orbit). And then there is the problem with the fragile gimbals needed for all the antennae.

In the end, yes, you can communicate via radio. No, it isn't broadband. No, it won't service a population comparable to the internet. It will likely be biased communication. It will still be expensive. There will be extreme needs that keep every carrier busy. So, although you COULD send a digital copy of Pluto Nash to Pluto, you would never get enough priority to use the carriers and thus it would be simpler to put it on a torch or fling it out the airlock.

First of all, welcome to some new commenters! And I am loving this discussion.

A meta disclaimer: Calculus defeated me twice. It wasn't the calculus so such, it was that I am a wretched puzzle solver.

Fancy algebraic expressions just totally lose me. The typical homework problem in intro calculus looks like Greeks faking hieroglyphics to me. And I end up aimlessly moving x's and y's back and forth without ever simplifying the damn thing to the point were you can apply a standard integral and solve the friggin problem.

Which is one reason I am not a real rocket scientist.

Torches and terawatts. The problem isn't so much producing a terawatt of power, it is getting rid of the terawatt or more of waste heat that you also produce.

Large liquid fuel rocket engines develop on order of 1 MW/kg of thrust power, which is about 100 times more than a jet engine of the same mass. But one byproduct of their huge propellant flow is that they can use it as coolant before burning it.

The fuel economy of a torch drive means it can't do this to any significant degree; you need some other way to handle the waste heat.

In principle, the simplest way to capture fusion energy for propulsion is to start with a large pellet, with a thick layer of cladding surrounding the fuel core. For travel across interplanetary distances you only need to 'burn' a small fraction of your propellant mass, as little as 0.01 percent. The rest adds no energy, just heft.

A large enough radius fuel pellet would moderate and thermalize most fusion neutrons, which is a double benefit - their energy is transferred to the bulk plasma as kinetic heat, which you can directionalize, and the neutrons themselves stay in the plasma and are carried off harmlessly in the exhaust.

But to achieve this you need an initial pellet radius on the order of a meter and pellet mass of a ton or so. (Very rough SWAG numbers.) Which means that your pellet energy release is in the kilotons.

This amounts to an advanced fusion Orion. If you want smaller pellets, which you do, the only way to capture most of the energy as kinetic motion would be to implode the pellet to some enormous density, presumably even beyond what is needed to trigger fusion.

I have not a clue how much energy this would take, but I'm sure it would be a lot, that would have to captured and stored from cycle to cycle.

Communications. This whole subject gets discussed far too little, so I am glad to see it come up here.

My intuition, worth what you paid, is that point to point comms bandwidth, probably by laser, will be surprisingly cheap even across interplanetary distances.

Very quick and dirty. If I am using the formula from Atomic Rockets correctly, a visible band laser beaming through a 1 meter telescope will have a beam radius of 90 km at 1 AU.

But an ordinary light bulb is visible at 45 km (in ideal conditions). It is shining in all directions, so the projected surface is the same, and a 100 watt visible band laser should be naked eye visible at 1 AU.

Anyone want to check this? A laser beam bright enough to be visible should be able to sustain a huge data bandwidth, and even a spacesuit helmet could probably carry the receiver. A 100 watt laser and 1 meter telescope aren't a huge order either, as such things go.

If all of this is more or less correct, high bandwidth space comms should be cheap. By my rule of thumb for a mature midfuture space tech, the transmitter rig might be a ton or so all up, and cost $1 million, no extravagant fitting for a spaceship.

Receivers could cost a few kilobux and fit on a spacesuit helmet, though high bandwidth interplanetary comms in your spacesuit is probably taking 'mobile Internet' to needless extremes.

Proviso also that all this applies only to tight beam comms, for which you need to know exactly where to point your telescope. Which makes precise locational data crucial at least to high volume comms.

"Proviso also that all this applies only to tight beam comms, for which you need to know exactly where to point your telescope. Which makes precise locational data crucial at least to high volume comms."

Reliable (read: up-to-date) ephemeral data will be in very high demand. I can imagine that the mathematicians keeping everything pointed in the right direction will be very well-employed.

90km at 1 AU means that your signal has dropped to like one ten billionths of the original signal strength. 90km at 1 AU is also a very small spec to aim at. Additionally, if the receiver isn't pointed at just the right angle, you get nothing. Plus it is hard to distinguish a visible laser from a star. The real trick is to use a wavelength that is not generally present in space. I just don't see the high bandwidth at that range.

Yeah I sort of feel guilty about linking to it at all. I was able to somehow wrench myself from this black hole of a website for several months and accidentally ran into it yesterday.

Also: sorry for high-jacking this post.

On subject: what about relays at Lagrangian points?

Everyone knows where those are, and if someone needs to connect they can just link into the network. I don't even think lasers would even be needed. The relays would have high gain antennas to receive the data on broadband signals, and when a ship or colony needs to link in they can query the closest available platform with a much lower-gain antenna.

If we can be in contact with various probes (like Voyager probes) at interplanetary distances, on 1970s technology, then I hardly think you would need a giant technological leap to create a system-wide comm network.

-- By the way, we were talking about NASA probes that refuse to die, I definitely nominate Grandpa Voyager 1 as a mind-boggling success. 43 years, and still transmitting scientific data, headed for the Heliopause, thirty light-hours away. Ladies and Gentlemen, a standing ovation please.--

Granted you won't have the bandwidth of fiberoptic cables, so no browsing for Earth-porn from Callisto, but I don't see communication as much of an impediment. The only real reason to use lasers (that I see) would be for private (read: military) communications that you don't want intercepted, but with a target area of 90 km in radius, it's not really very private anymore.

Surfing the interplanetary web would be a unique experience. Given the reply times measured in minutes or hours, getting a response for a search query would be more akin to making a request at the Library of Congress and then waiting for the attending librarian to go and bring you a copy of whatever they think you asked for (which may or may not be what you were actually looking for).Since we survived the dark ages of dial-up modems, I think we could probably put up with the connection speed of the interplanetary net.

I think that interplanetary communications networks would be more like a cluster of 'webs'; one on Earth, one on Mars, one on Luna, ect. The different webs would send updates to each other on a regular schedule via dense-data/high-priority channels, and all other inquries/messages being sent via lower-priority channels. Most of your routine web activities would be with your local internet, but occationally you'd connect with another world's internet via the systen-wide web...

Interestingly enough, this is the way the "extranet" is postulated to work in the Mass Effect games. Essentially all colonies pack with them a web server full of the most needed and demanded data. If the information you are looking for is not on the server the system opens a communication link (FTL in the game) and hits the closest large colony server, and so on, until it finds the info, and it uploads it into the colony's server for easy retrieval later. In the game data storage is no longer an issue, but I assume if storage space is needed and a specific file hasn't been looked at for a while, it would be deleted.

However, since most early colonies will be science bases, I would assume large amounts of data will be passed back and forth as scientists on Callisto and on Earth look over the data, form theories and send them back and forth. It seems like a very fluid form of data exchange is needed even with local "nets". I also doubt early colonies will be very large, or widespread, and it is far more likely the Callisto colony terminals will be linked on a LAN to a single server rather than a network of servers in the first place.

Using relays Jean Remy mentioned would indeed solve problems in aiming the comm laser. And these could be located not only at Lagrangian points, but on stable orbits around the Sun. Just have enough of them so there's at least one between you and the Sun at any given time. When the orbits and positions at some point are known, the positions at any given time can be calculated.

@Citizen Joe:"Plus it is hard to distinguish a visible laser from a star. The real trick is to use a wavelength that is not generally present in space. I just don't see the high bandwidth at that range."

I don't know enough about telecommunications to say much about bandwidth. But why couldn't there be a high bandwidth at frequencies not generally present in space?

The following is speculation and may not be anywhere near the truth. If so, please tell me.

I understand that data transmission rate is effected by the signal rate (with a laser, I believe that's directly related to the rate you can switch the thing on and off, producing a stream of binary information) and the bandwidth (with a wider band of frequencies you can transmit multiple bits at once, each at its own frequency). But what is the exact relationship between these? Is it just:D = R*Nwhere D is bits/second, R is the signal rate in hertz and N is the number of bands (frequencies, wavelengths) you are using. In this case, if you have a transmitter with a bandwidth of 1 kHz, wouldn't you technically have an infinite number of frequencies? You could transmit at 1 Hz, at 1.000001 Hz, at 1.000002 Hz et cetera. It would be only limited on how finely you can divide the bandwidth.

Viral propagation is another option. It doesn't guarantee speed or privacy. The idea would be that a message would be sent to any ship that is heading in the right direction that is within range. This could be a very long route. However, since there would likely be relatively few interplanetary vessels (compared to airplanes), Solar space traffic controllers would have the full list of vessels and thus able to chart a route based on predicted paths. While the InterPlanetary Space ships would carry parcels, they would probably also serve as communication hubs for the viral network.

"Viral propagation is another option. It doesn't guarantee speed or privacy. The idea would be that a message would be sent to any ship that is heading in the right direction that is within range. This could be a very long route."That sounds pretty close to how the modern internet functions, with data tracing geographically indirect paths as it goes from place to place. I like the efficiency of such a scheme: it would use the infrastructure that's already there to create an ad-hoc network backbone. On the other hand, you'd be trading out data security unless you've got some very good encryption (or just plain don't care who else reads your messages).

re optical signals being crowded out by stellar background, I'm no optical engineer, but I don't think this would be an insurmountable problem either. Terrestrial laser-guided bombs manage to find a tiny blinking light under daytime conditions (I think they use a fancy set of optical and digital filters to sort out the signal they're looking for). Also, the images from our optical and radio telescopes tell me that the astronomers have figured out how to block out signals they're not interested in. IMO the solar system could probably handle quite a lot of data traffic without things getting too fouled up.

@ Jean Remy,I was thinking along those same lines. A lot of business/corporate/government entities would end up hosting proxies of their sites and databases on the far-flung colony servers, with periodic updates being beamed back and forth (ultra-secure data being handled differently).With multiple proxies being hosted for a single database, with data exchange rates measured in minutes, how long do you think the system will last until Murphy's Law takes a server down through version conflicts?

Ultra high bandwidth communications will be easy enough between "base stations" which can support not only powerful transmitters but also huge receiver stations (large antenna arrays, photodetectors, neutrino tanks or whatever tech we care to use). Finding a planet, asteroid or Venus Equilateral should not be too difficult, and tracking demands will be quite modest.

Moving spacecraft will be a lot more difficult to accurately track with a tight, high bandwidth beam, and they will generally not have such large scale transmitter or receiver arrays. The two exceptions would be a laser battlestation, which would simply use the targeting array and laser (dialed down to a suitable frequency), and torch ships under weigh, which can be tracked by their drive emissions. Of course putting a transmission beam through the drive plume might be difficult in itself.

Ships would probably have more need for local high bandwidth communications between ships in a constellation, or to constellations of sensor drones around the ship itself.

Good point, Thucydides.Ships en route wouldn't have the means to relay a huge amount of data.Perhaps station-ship-station relay networks would be more common as the system is just starting out, and later on only out in the boonies. Data routes with a high demand would soon get their own dedicated infrastructure.One other interesting feature of intrasystem communications: the transmission-time "tides" that everything (minus parent-satellite combinations) is subject to. Mars would cycle between a 3-minute delay and a 20-minute delay for getting a signal to the earth-side servers, with its tidal cycle taking more than a year to complete. It'll be interesting to see how people deal with the variability. Here, if you purchase internet access you can expect the *average* connection speed to stay constant.

Viral dissemination works on Earth because of thousands and thousands of privately owned servers.

However, even in the best-case scenario of a very developed interplanetary infrastructure, I don't see a lot of traffic in space. Say two cyclers between every major colonial epicenters (say 2 for Mars, 2 for the Jovian colonies, 2 for Saturn etc...) and a few "moon hopper" shuttles, but those would be so close to their giant primaries getting a Line of Sight on them would be an issue.

However the Comm relay platforms suggested are basically Voyager probes without the scientific instrumentation and a known stable orbit. Rather that throwing your message out omnidirectionally and hope that eventually it will reach your destination (because viral dissemination is kind of like shooting blind) you simply bounce the signal of a set number of known (and if want, secure) predetermined platforms. If your goal is to reach as many people as possible (the entire point of viral dissemination in the first place) then target the Cyclers. Chances are good the passengers en route back and forth have personal computers linked in to the ship's server, which keeps updated by linking in to the platforms.

Re: communication speed. I'm not an electrical engineer, so the following calculations may have grievous errors, but I'm going to try to throw some numbers out and see what sticks.

Let's assume a spread-spectrum laser communicator, using Rick's assumptions of 100 W and a 1 meter aperture at 1 AU. We choose a frequency spread of 125 THz centered around 1 micron infrared (roughly corresponding to 0.8 micron wavelength to 1.25 micron wavelength). We will use a 1 square meter aperture telescope to receive the signal, focused onto a detector cooled to 4 K.

This laser will deliver 4 nW/m^2 at 1 AU, so we pick up 4 nW of signal. The thermal (Johnson) noise of the detector is roughly the temperature (in energy units) times the frequency spread. 4 K is 5.5E-23 J, so we have about 7 nW of noise. The Shannon-Hartley theorem says the maximum rate of data transmission is equal to the bandwidth times the log base 2 of (1+(signal)/(noise)). Using log_2(1+4 nW/(7 nW))=0.65, we find that we can move data at a rate of about 80 THz under optimum conditions using optimum engineering.

An interesting thing I just noticed - in the limit of low signal to noise and assuming only thermal noise, the rate at which you can transmit data is independent of your carrier bandwidth. This is because log_2(1+S/N) = ln(1+S/N)/ln(2) ~= (S/N)/ln(2) ~= (S/(TB))/ln(2)where S is the signal power, N is the noise power, T is the temperature (in energy units), and B is the bandwidth. Plugging this in to the Shannon-Hartley theoremR = B log_2(1+S/N)for R the rate of data transmission, we can see thatR = S/(T ln(2)).In other words, the rate of data transfer depends only on your signal strength and noise in the detector. Ultimately, this is because the noise you pick up decreases linearly with bandwidth. Using a narrow-band signal also cuts down on background noise in that band. Then all that matters is dumping enough power into the signal to be heard.

Luke, Marcus, et al...all that is correct if you're using Frequency Division Multiplexing (FDM), but with Time Division Multiplexing (TDM), you can get a much higher data rate from the same bandwith...Polarizing Division Multiplexing also has a higher data transfer rate than FDM...Composite Multiplexing (CM),the combination of two or more of those, would give you a much higher data transfer rate than any single multiplexing technique. Modern-day laser communications can carry millions of channels of data in a single frequency beam; I see no reason that a resonable data rate, even at interplanetary ranges, couldn't be fesible between Earth and a theoretical Callisto colony in as little as 50 years or even less. A time lag of one to two hours might be a bit long for 'chatting', but fine for email. If you need some vital, highly-sensitive information, but not right away, then a torchship might fit the bill as a fast packet or mail ship.

@Luke: you assumed a 4 kelvin receiver. I believe this would require active cooling, especially on the side which is turned towards the Sun. Assuming radiative equilibrium, the temp at Earth orbit (semi-major axis) is approx. 390 K, at Mars orbit it is 320 K. These would produce some 100 times the noise, reducing the bitrate from 80 Tbps to 1 Tbps if cooling was not used.

Still, that's pretty impressive with a 100 watt laser. Presumably the relays would have more powerful transmitters, since they don't need many other systems, being automatic and orbital.

Now, if there was a conflict in our solar system, could communications be jammed? One could blind the relays and cut off communication to half the system. Laser battlestars could perform this role with their multi-gigawatt lasers from great distances away. Could you defend against this kind of jamming? Would anything get through if an opponent was shining the receiver with 1 mW/m² while you're trying to reach it with 10 nW/m²?

Much good information as usual, and in balance it confirms my sense that interplanetary communications will be (relatively!) cheap and plentiful.

Though it may not seem that way to people accustomed to Earth bandwidth, because current experience suggests people will use as much of the stuff as is available.

People also put up with some amazing inherent limitations, like watching TV on an iPhone.

Most ships won't need interplanetary comms, because they will be operating some region of orbital or local space, and can relay through local stations. Perhaps only deep space ships will carry such gear, and they are their own special beast anyway.

But the fact that high bandwidth interplanetary comms is not THAT hard or expensive is politically significant. As so often, technology has ruined or at least limited some rocketpunk era tropes, in this case 'seizing the relay station.'

It may have killed a trope or two, but it looks to me like we've opened up a few interesting possibilities.I for one am not quite sure how a station or ship would deal with its comm laser module getting blinded by an enemy laserstar. Since Markus brought up that possibility I've been wondering how someone could engineer around that. You could deploy a thermal shield, and I suspect comm lasers would already do that as a matter of course. However, that could be overcome by a more powerful laser or one from the same direction as incoming transmissions, which could be countered with a better optical hood around the receiver aperture, ... etc.Also a thought on data security (Jean Remy actually mentioned this in passing earlier). Since that hypothetical transmission beam we've been throwing around has a 90km radius after it's gone 1 AU, there isn't much to keep someone from listening in. If the intended receiver was out in the middle of deep space, it would be easy to catch the eavesdropper, but if it were happening in crowded planetary space, how do you tell who's listening in and who's just cruising by?

At 1 AU, 90km is puny... almost a point. In order to catch that dart, you'd have to be pretty close to the intended destination... like 90km close. If you were to try intercepting at the mid point, you'd need to apply thrust for station keeping since your orbital speed wouldn't match the source or target. That would make you very obvious. Also, if you were to pass through, you would likely only dwell in the beam path for a few seconds.

And I found the error (I think). When you switched from visible light to THz radiation, the beam spread went from 90 km to 183 THOUSAND km. That drops your capture radiation by a factor of 2.5e-7. That in turn drops your bandwidth to basically zero.

I think you'd do better with something in the UV band, although they are harder to focus and I think there is a lot more UV noise in space.

I was under the impression that Luke's calculations gave the information flow rate in THz, not the carrier frequency, though there could have been a swap somewhere in the math (EM physics is not my strong subject).

When I mentioned eavesdropping I made the assumption that if the receiver is in low orbit around a planet, that receiver would probably be attached to a ship or station that itself was surrounded by other ships/stations also occupying that orbital space. In such an environment, the 90km that is a pin-prick in interplanetary space wouldn't seem so tiny anymore. I think this is the only place where eavesdropping would be possible at all. It's much more plausible that a ship can get within the 90km-radius signal-cylinder containing the receiver in these circumstances, rather than trying to do it in the great Out There.You'd actually be likely to see eavesdropping on comm signals work only in the same circumstances that orbital piracy would work: a crowded environment where everybody's packed close together (compared to the scale of space) and it's harder to keep track of everything around you.

Re: Comm jamming. My guess is that you would use directional information to discriminate between sources. Use a telescope, and only measure the signal from the pixels corresponding to the guy trying to communicate with you. A powerful enough laser in the same field of view could have diffraction spillover into the image of the talker, but this would require the laserstar to be nearly lined up with the talker and the receiver. Of course, a laserstar could also just burn the comm scope off, it were within range.

Re: Cooling. Yes, I assumed refrigeration down to liquid helium temperatures. I figure an extra 250 kg of helium liquifiers and cryostats and compressors and helium pumps is a small price to pay for a 50-fold increase in data rate, especially with cheap, abundant solar power. In any event, by the time orbital commerce becomes commonplace we'll probably have compact solid state chillers that can get us down to 4 K with less than a kg of mass.

Citizen Joe: When did we start discussing THz waves for communication? In an odd coincidence, the folks on rec.arts.sf.science started discussing the power requirements for interplanetary communication at the same time as Rick did here at Rocketpunk Manifesto, and I used the same analysis there but for a THz microwave beam (although I accidentally put the ln(2) in the numerator rather than the denominator).http://groups.google.com/group/rec.arts.sf.science/browse_frm/thread/0e0b49a817ab3d49/7b8b61cf89844055?hl=en#7b8b61cf89844055I concluded that a 1 THz, 1 kW beam from and to 1 meter apertures with the receiver cooled to 4 K could, at the engineering best, get about 45 MHz bit rates (a bit better with the ln(2) in the right place, but I'm neglecting much larger errors than that in these analyses, so I'm not going to worry about it).

I hadn't even noticed that Atomic Rockets didn't have anything to speak of on communications. Come to think of it, though, I don't remember seeing anyone mention interplanetary communications in any treatment of sci-fi outside the realm of magitech(TM).

Luke, I'm curious about that hypothetical laser comm you crunched the numbers for. At close range (i.e. close enough that there's not much beam spread), would you be able to damage anything (that's worth damaging) by shining that laser on it? This is echoing the question/notion posed much earlier by Stevo Darkly.

Eric: I used Rick's parameters for a lasercom, which was stated as 100 W of power. A 100 W laser can be used at close focus in the near field range for industrial cutting, drilling, and welding. However, it is generally considered that you need three or more orders of magnitude more power in order to have a useful weapon.

On another note, I was thinking about shot noise. That 4 nW of 1 micron laser light corresponds to 20 billion photons per second. This alone limits you to no more than 20 GHz. If we assume that the laser transmits pulses that are either on or off, and we assume the absence of any background, then you need an average detection of 14 photons for a 1-in-a-million chance of a false negative (picking up zero photons during the "on" pulse). This reduces your data transfer rate to a bit more than 1 GHz. If we assume a 1 GHz bit rate (an average of 20 photons per pulse), you can set your lower limit of acceptance to 3 or more photons to take into account background or detector noise, and still achieve a bit error rate of 1-in-2.5 million.

Note that for laser light, shot noise puts a much more stringent limit on communication rate than thermal noise.

In the rocketpunk era there was quite a bit about communications tech. Radio comms were a plot point in at least a couple of the Heinlein juveniles. In Rolling Stones the protagonists' space yacht uses a large space liner in nearly the same orbit as a relay so Roger Stone can send his space opera scripts to the producers on Earth.

Odd little side note. When I was growing up, serials had already vanished from 'Murrican TV, and standalone episodes were universal. The continuing adventures of Stone's hero seemed quaint. But continuing story arcs are pretty much the rule in present day TV SF, so we have pretty much gone full circle.

Bit errors are fairly tolerable in the highest bandwidth applications, such as video. The bandwidth of 'every bit is critical' data is usually much lower.

(I'm talking about general applications here, not surveillance video where one bad bit might hide what you are looking for.)

Rick "We are biased toward transportation, which is much more Romantic. Hence the common ploy of having no FTL comms, or only rare and expensive, even when FTL starships are cheap and common."

L.M. Bujold had a good point in her Vorkosigan series. In that ficton many solar systems are connected by FTL 'wormhole jumps'.

At one point it is mentioned that in high traffic areas of the 'wormhole nexus' radio or lasers is used to send information between wormholes that are in the same solar system & then a ship jumps through the wormhole every so often to relay the information through the next solar system.

This gets information through much faster than carrying it all the way on one ship, even though they are torchships.

as far as comms, wouldn't you want to have relays closer? For example, if you have colonies throughout our solar system, and haven't expanded humanity beyond that, perhaps you would have a relay station in orbit around each gas giant. If you were on a ship or colony near Pluto, and wanted to send a message to Earth, you would not transit directly to earth, but instead to Neptune, and that station would relay it to Saturn, that to Jupiter, etc. You would only transmit directly to Earth if you had a top secret report. So, most communication between Pluto and Earth would be cheap, once there was an established human presence throughout the solar system.

Pluto could easily be on the opposite side of the solar system from Neptune. You could drop in relays at the L3-5 points of Jupiter. That would minimize the relay distances.

As a side note, in the space opera setting I worked on, the aliens had FTL comms via micro wormholes, but the humans never developed the technology. Instead, due to long relay times, humans developed advanced AI's that would make the moment to moment decisions.

Given the human mind can detect patterns and come to conclusions based on very limited information (not necessarily the correct patterns and conclusions, mind you), we might not need high bandwidth communications as much as we seem to believe.

I am guessing that most colonies would be pretty much autonomous and probably operate based on the shared assumptions that the early colonists brought with them. (For a macro version of this hypothesis, read Samuel Huntington's book "Who are We?). Most of the high bandwidth information isn't for the consumption of people but rather "machine" data for systems which do not discriminate patterns or arrive at conclusions very easily. To use a military example, imagine an Aegis missile cruiser. Huge amounts of data are flowing through the sensor systems and being processed and projected on various screens. The systems are cycling in the gigahertz range while the watch officer in the CIC is absorbing the data at mere human speed. Despite the disparity between the data transfer rates (and the fact the officer probably isn't consciously absorbing more than a fraction of the data anyway), the fire/no fire decision is in the hands of the officer, not the computer.

Until HAL 9000 and his pals come along, I suspect this state of affairs will continue in many elements of industry, science, finance and other human endeavours. Most high bandwidth for human consumption is in the form of entertainment (after all, wars and international commerce were possible even when communications was by clay tablet).

The general outlines of communications in the plausible mid-future look a bit like the Victorian era. Fast and reliable close to home, a little slower if you want to contact the next 'city' over, slower again but still reliable if you want to contact another 'country', and best of luck to you if you want to contact someone in the backbeyond. In this case city, country, and backbeyond are defined by both linear distance and orbit.

You could even end up with a Pony Express situation. Someone sets up a Planet Express torchship delivery system to carry high information-density packages or mail that requires high-security delivery... And then nine months later someone else sets up a tightbeam relay network that can handle multiple high-bandwidth high-security messages... And suddenly you've got a lot of cheap ponies on the market.

nitpick: When I was worried about the 90km spread of the beam, I didn't imply that you had to be withing 90km of the receiving ship to eavesdrop. You could, from the standpoint of the emitter, be within the separation angle that corresponds to 90km at the receiver's point, yet still be a few thousand kilometers closer or further on a direct line to said emitter.

What I really meant was that you could casually stroll *behind* the receiver by several thousand kilometers and still catch the signal. That way you don't spook your target by coming between it and the emitter, and you are still able to eavesdrop.

Once again, due to orbital speeds and different orbits, in order to stay within the loiter zone of the beam transmission, you would have to apply thrust. That makes you obvious, particularly because you're loitering within the line that the two ships are staring.

For long distance communications where the light speed delay is a hour or more (like Jupiter), information could be sent in redundant packets. So, break down the whole message into shorter packet sizes. Then transmit each packet three times. A modest expert system then compares the three packets to decide whether there is lost data and which packet or portion of the packet is true. If the confidence in the transmission is low enough, it would only then ping back for a re-send... which would be at least a two hour delay. The whole message itself could take only a couple minutes to transmit. Since the delay is so much longer than the actual transmission time, it would be feasible to reduce the transmit rate to improve the gain and thus confidence on the other end. So transmitting to close Mars (.5 AU) might be 1000 packets per second, while far Mars (2.5 AU) may only be 200 packets per second. Jupiter would be like 100 packets per second and Pluto would be 25 packets per second. The packet rate would be based on the expected confidence of receiving the data at the other end. Since ships are moving about unpredictably, their packet rate could be as low as one or two packets per second.

I agree with Citizen Joe. To loiter for any period of time you'd need to take a specific orbit, and the loiterer's presence would be a sure tipoff.

Exception for ships in similar orbits, but generally, it is hard to intercept a tightbeam without active measures. And the people talking can always save sensitive information for when nothing is lined up with the beam.

Its awesome how this entry turned into a discussion about interplanetary communications.

In putting it all together I am trying to figure out how one can have a hard science fiction novel yet still have a space opera with the different star systems and their different races. I originally started a novel trying to stick to a hard line but even at near light speeds space is a b^tch: I caved to FTL: my main character would have been dead long before anything happened: I supposed it would be interesting to have an interstellar conflict that spanned hundreds if not thousands of years but how could such a conflict even begin let alone be maintained? By the time your ships armed with lasers/nukes/kinetic weapons etc. reached said star system your political enemies may already be dust.

Of course you could go the wormhole route to maintain an ounce of scientific credibility, but isn’t that all it is: an ounce? Something about wormholes just goes against my primitive brain, perhaps it’s the negative mass/negative energy thing. There was a quote I read, I can’t seem to find it but it was something like ‘wormholes are like jumping off the Sears Tower and expecting to end up in the Columbia Center.’

Say there are two interstellar empires whose home worlds are (conveniently) 30 light years from each other, each have terra-formed planets that are (conveniently) 20 light years from each other, colonies that are (conveniently) 10 light years from each other with outposts that are (conveniently) 5 light years from each other. Ships can reach .6 c (1.275 relativistic effect) for a transit time of 8 1/3 years (not including accelerating and decelerating time) from outpost to rival outpost, 16 2/3 years from colony to rival colony, 33 1/3 years from terra-formed world to terra-formed world, and 50 light years from home world to rival home world.

Negotiations take place in between outposts and communications as a result of these negotiations take 2.5 light years to get back to the outposts, 5 light years to get back to the colonies, 10 years to get back to the terra-formed planets and 15 light years to get back to the home worlds. The response from each home world?: 5 more light years to get back to the terra-formed planets, 10 more light years to get back to the colonies, 12.5 more light years to get back to outposts and 15 more light years to get back to the meeting place. So 30 years goes by from the initial disagreement to the response from the central government. How can you have a ‘Star Wars’ like conflict under these circumstances (add a handful or more interstellar nations to this scenario)! Pizarro and Cortez didn’t even operate under similar circumstances, and if they did. . . Even if you halved the distances your talking about a 7.5 light year communications lag from home planet to meeting place; an Alpha Centauri lag from an in between meeting place: 2.185 light years. The Captain/Commanders at the outposts would have to be kings unto themselves, and if one of these viceroys’ egos got out of hand.

[This is as much math as you’ll see me doing; and I probably bungled it up!]

I have a suggestion for The Rocketpunk Manifesto: with all of the apparent knowledge here perhaps TRM and its commenters should pool together to make its own hard edge sci-fi novel. I think we would need a decision-making hierarchy: I nominate Rick as Dictator: The RICKtator: it is his blog. I have read some of Orion’s Arm’s stuff (anybody else?) and it seems pretty interesting, but I became relatively uninterested when I came across the ‘Siberoo’ (though I am starting to warm up to it a bit), plus its universe is centered around human advancements, or more appropriately the advancements of the human’s advancements (ei: no funkalicious aliens). I would be interested in seeing what The Rocketpunk Manifesto could come up with: anybody else?

VonMalcom: I will mention that we already know of at least two cases which are experimentally verified as having negative energy density - the Casimir vacuum between conductive surfaces and so called "squeezed states". If black holes exist, then the event horizon of a black hole will also have a negative energy density.

One nice thing about wormholes is that they let you adventure in a universe filled with interesting aliens that are naturally neither so god-like in their technology that they completely out-class you nor mere stone-age primitives.

Consider - suppose we humans invent a way to split off a pair of connected wormhole mouths from the vacuum and keep them open. We can use them for interstellar transport by charging up one of the mouths and putting it in a particle accelerator to shoot it out toward an interesting looking star at ultrarelativistic speeds (make sure to discharge it in flight, or it may be deflected by interstellar magnetic fields). When it reaches the destination star, slow it down by shining an intense laser through it and using the light beam as a photon rocket. Once you stop, gobble up some mass so you can send things through.

Now, the thing about wormholes is they do not connect points in space, they connect events in space-time. That ultrarelativistic wormhole you shot out will have a very high time dilation while it is in motion. From the point of view of the wormhole mouth in motion, it might only take a month to make a 100 light year journey due to time dilation. Since the wormhole mouth back home is connected to the wormhole mouth in transit both in space and time, the people back home only need to wait one month before they can look through the wormhole and see the virgin star system, ripe for colonization. We'll call our new conquest Terra Nova.

Of course, in our reference frame that is not looking through the wormhole, it takes somewhat over 100 years for the wormhole mouth to travel those 100 light years (for the listed time dilation, it takes 100 years, 18 minutes). This means the wormhole is a time machine that takes you (roughly) 99 years, 11 months into the future if you go from Earth to Terra Nova, or 99 years, 11 months into the past if you go from Terra Nova back to Earth.

Now there are certain details we will need to follow if we have wormholes to many star systems, to prevent the creation of time machines (which will probably break the wormholes involved before we can make the time machines). The main idea, though, is that an expansion front of earth civilization sweeps through space at almost the speed of light - and due to time dilation, as the expansion front overtakes regions of space, they are linked back to human civilization at a time (and thus level of technological advancement) not too far beyond what is needed to make wormholes.

Now, suppose there is another technological civilization in a distant galaxy. Maybe they have not even evolved by the time we start sending out wormholes (in some galaxy centered reference frame). Maybe (in that galaxy centered reference frame) they were ancient long before our distant ape-like ancestors came out of the jungles to gaze across the African savanna. Nevertheless, due to time dilation effects of wormhole transport, when our expansion front meets their expansion front, we will both have only recently invented wormholes (well, maybe within a few hundreds of years - but not millions of years).

I was reading on the Casimir Effect before posting my previous comments; Wiki stated: 'The closest known real representative of exotic matter is a region of pseudo-negative pressure density produced by the Casimir effect.'

I didn't/don't understand how close of a representation this 'pseudo-negative pressure density' was/is to negative matter and/or negative energy: if it was/is a play on words, an effect that mimicked negative mass/energy, an actual stepping stone, etc. . .

You don't need negative matter to prop open wormholes, just a region with a negative energy density.

The space between two conductive plates has a negative energy density (this is the Casimir effect).

Thus, we demonstrate the existence of an effect that is needed to prop up wormholes.

Unfortunately, the Casimir effect is very weak. Relying on the Casimir effect to build wormholes is likely to be impractical, unless you have many star systems worth of mass to play around with. What we need is something else that has a negative energy density but is a lot more significant than the Casimir vacuum between the sorts of polished metal sheets we can make in the laboratory.

Von: Had a little trouble following your math because you kept using light-years to mean years, but once I got over my headache and made the substitutions it seems solid. Yes, with communication lags like these interstellar war is not possible. It is remotely possible (though unlikely) that we'd send a sleeper or generation ship to colonize another world, but that new world would be completely separate from Earth in terms of political authority, and out of military reach.

Luke: trying to wrap my brain around the wormhole but wouldn't it be the opposite? That is relative the the wormhole the travel time is one month but from the standpoint of the people who launched it (and aren't moving) the it takes 100 years for the wormhole to arrive at its destination, at which point any traveler going through it would be sent 99 years 11 months back into the past, that is to say 1 month after the wormhole was launched, so still in the future with regard to the launch date (no time travel to a time before the wormhole is initially created) and maintains causality because no message can get back from Terra Nova to Earth in less than 100 years, and therefore 1 month after the wormhole was launched...

Luke: wait no I think I've got it. The wormhole is not 100 years old when it arrives, it is only 1 month old (so we have to find a way to maintain a wormhole open for one month, but... gah! need more thinky time.

I think the problem here is that there are two conflicting reference points on either side of the wormhole's event horizon. To get the one hundred years into the future leap you would have to be within the event horizon at the moment you launch the other end through space at fractional-c.

Say you can only maintain the wormhole open for one month, you built a ship within the even horizon, launch the extremity, which arrives in 1 months time (since you are in the reference point OF the wormhole) at which point you go through, find yourself one hundred years in the future and the wormhole collapses.

However to anyone standing outside the event horizon, the entire thing collapses in a month and you can't even get any information from the start system the wormhole reached because... damnit.

Wormholes are only one way then. You can send information and matter through the origin endpoint to the arrival endpoint, but not back, because information cannot cross that even horizon in that direction. Interstellar travel back and forth would require a pair of wormholes.

VonMalcom: If you can find matter with negative mass, that would certainly work! There are fairly strong indications, however, that any region of spacetime with negative energy density must be bounded by a region with positive energy density such that the total energy is positive (and there exist certain bounds placed on how negative and over how large a region the negative energy region can be based on the extent and magnitude of the bounding positive energy). If this is the case, whatever it is that has the negative energy density doesn't seem much like matter with a negative mass (for one thing, if you move it out of its positive energy "container" it vanishes). All examples of negative energy density with scientific support satisfy these bounds.

Jean Remy: Perhaps a timeline would help. I will use GMT to refer to the Greenwich Mean Time coordinate frame. Keep in mind that the actual time coordinate depends on your frame of reference.

Jan 1, 00:00:00.00 2050 AD GMTMankind launches a wormhole mouth toward Nova Terra. The other mouth remains on earth. Nova Terra is 100 light years distant from earth. The launched wormhole mouth has a time dilation factor of 1200 - for every second of proper time experienced by the mouth, 1200 seconds pass in the GMT coordinate frame. To make this explicit, a motor is placed inside the wormhole. The motor turns a drive shaft that connects to an analog clock face on each side of the wormhole. Since the shaft turns at the same rate for both clock faces, anyone looking through the wormhole sees the same time on both the clock face on Earth and the clock face on the other side of the wormhole. The clock drives the shaft at a rate such that the clock faces turn at one second mark per second of proper time. A time dilation factor of 1200 corresponds to a speed of 0.999999653 c.

Jan 1, 00:18:15.75 2150 AD GMTThe wormhole mouth arrives at Terra Nova. 100 years, 18 minutes and 15.75 seconds have passed in the reference frame at rest with respect to Earth. This is 3,155,761,095.75 seconds. Due to time dilation, the projected wormhole mouth experiences only 1/1200 of this of its own proper time (equivalently, time in its own inertial coordinate frame). This means the proper time of the projected wormhole mouth is 2,629,800.91 seconds, or 30 days, 10 hours, 30 minutes, and 0.91 seconds. Anyone who had been drifting along with the wormhole mouth would have experienced a passage of time of 30 d, 10 h, 30 m, 0.91 s. If she were watching the clock, she would have seen it tick off that amount of time. Since the clocks on both sides of the wormhole are ticking along at the same rate from the point of view of someone looking through the wormhole, anyone sitting back on earth watching the clock would have seen it tick off 30 d, 10 h, 30 m, 0.91 s. This means that 30 d etc after launching the wormhole, people on earth experience the wormhole's arrival as viewed through the wormhole. This then means -

An explorer going through the wormhole the moment it arrives would go from a time coordinate of Jan 30, 10:30:00.91 2050 AD GMT to a time coordinate of Jan 1, 00:18:15.75 2150 AD GMT. This is a jump forward in time of 99 y, 334 d, 19 h, 48 m, 14.84 s. If one of the little green native inhabitants of Terra Nova were to jump through the wormhole the moment it arrives, he would go from a time coordinate of Jan 1, 00:18:15.75 2150 AD GMT to a time coordinate of Jan 30, 10:30:00.91 2050 AD GMT, a jump backwards in the time coordinate of 99 y, 334 d, 19 h, 48 m, 14.84 s.

Luke, I think you just created a negative energy density around my brain with your last comment and it just got sucked though a wormhole. I read your comment once, I read it twice, I read it thrice: way over my head!

The colors are pretty though inside this wormhole: I've just gone plaid!

Luke: I got the gist of it the first time, I wasn't arguing with the mathematics which are excellent, but with the question of space-time referent, since we have to use special relativity to get an absolute point of reference (because there is none in general relativity) The problem I see here is that the duration of travel cannot be the same for the object in motion (head of the wormhole) and the objects at rest (Earth and Terra Nova).

If your calculations are correct then the time dilation occurred for both wormhole AND Earth with relation TO Terra Nova, which would imply that Earth was moving at relativistic speeds in the absolute frame of reference vis-a-vis Terra Nova...

Not sure if I am being clear here, but in your construction both the wormhole and Earth have to be moving at relativistic speeds in Terra Nova's absolute reference frame for the dilation to affect both equally. In your theory Earth and the wormhole that is moving at relativistic speed belong to the same frame of reference, but that cannot be because the wormhole head is traveling with respect to Earth's reference frame at relativistic speeds.

I think I am going in circles here. The point I am trying to make is that you imply that because Earth is connected by a cam to the end of the wormhole they are in the same reference frame, which they cannot be because the wormhole is traveling at fractional-c speeds with relation to said frame.

One of the problems with this theory of your is that I believe such a wormhole cannot work both ways because it would break causality, and that information can only flow one way through a wormhole. From one theory I recall reading, that is. It specified that two way space travel meant throwing the wormhole out, going through it, and throwing another wormhole back to make the return trip, otherwise you'd break causality. I am not sure about this, but if this is true you cannot have a cam linking the clock faces on both sides as this would imply sending information back and forth.

Jean Remy: Ah, I see a bit clearer now. Let me see if I can address your points.

First, keep in mind that a wormhole is, by its nature, a general relativistic object. The reference frames in flat spacetime from special relativity should not be expected to hold in the highly curved spacetime of a wormhole. I've tried, as much as possible, to avoid the curvature of the wormhole and use only observers located in spacetime that is mostly flat (i.e., on one side of the wormhole or the other) so as to be able to use special relativity to analyze the motion. However, you do need a coordinate patch at the wormhole - although spacetime across the wormhole is continuous, the specific coordinates that you use in flat spacetime will become discontinuous across the wormhole (alternately, you can choose continuous coordinates across the wormhole, but then you need to patch your coordinates together someplace else, creating a discontinuity in the coordinate representation between Earth and Terra Nova.

The key point is that the wormhole mouth en route to Terra Nova is both at rest with respect to Earth (through the wormhole) AND moving at relativistic speeds with respect to Earth (through flat spacetime). Likewise Earth is both at rest with respect to Terra Nova (through flat spacetime) AND moving at relativistic speeds with respect to Terra Nova (through the wormhole). The perceived speed is path dependent in this particular spacetime geometry.

Note that for just one wormhole causality is not broken. At Terra Nova, you can go back in time by 99 years, 11 months by going through the wormhole to Earth. However, you can never get back to Terra Nova before you started. If you go back through the wormhole, you will go forward in time by 99 years, 11 months, so when you add in however long you spent on Earth, you get back after you left. If you try to go back to Terra Nova the long way through flat spacetime, it will take at least 100 years since Terra Nova is 100 light years away - even if you sent yourself a lasercom signal to Terra Nova as soon as you got to earth, the message would not arrive until a month after you left. We maintain time ordering, and causes always precede their effects.

Out of convenience, it is often useful to consider a specific kind of wormhole called a Visser wormhole (after its inventor, Matt Visser). A Visser wormhole is essentially supported by a "cage" or "circle" of negative energy stuff, and paths through the wormhole that do not touch the cage only go through flat spacetime. Thus, any trip through a Visser wormhole is no different from traveling through flat spacetime. Visser wormholes are valid solutions of Einstein's equation for the geometry of spacetime in general relativity. This makes them convenient for analyzing cases like this - the flat spacetime through the wormhole no more impedes the flow of matter or information than any other region of flat spacetime, like the spacetime between my library and my living room.

I don't think there is time dilation occurring since there is negligible motion occurring, instead two points in space are connected with a wormhole.

We 'know' that space is warped by matter (gravity). But is space itself dragged along? Or is it ridged but flexible? If it is ridged, the wormholes will drift (rapidly) away from their sources. If space moves along in the wake of large masses, then wormholes might be more stable until they slowly drift out of the wake (or move far enough away that they collapse). So, in order for wormholes to be useful at all (to physical beings) space itself would have to be dragged along with matter. And unless wormholes are created by something other than matter, there would be insufficient time to create one before the conditions move away.

The property of matter that 'grips' space is mass. While the rest of the properties of matter deal with how matter relates to itself, mass relates it to space (and only tenuously at that). Talk to any physicist and he'll tell you that gravity is a weak force. So research into (useful) wormholes is going to involve 'grabbing space' so that the wormhole doesn't just wander off. If you can 'grab space' then it is relatively easy to make a reactionless drive by grabbing multiple points and then pulling. Of course, that won't be particularly fast (I'd say painfully slow) since your speed would be limited to the rate that you can 'grab space' much the same way falling from a mountain is different from climbing down.

Citizen Joe: If you accept general relativity as a useful description of the macroscopic universe, then the conclusion is inevitable - time dilation does occur. This is obvious if you do the math. If you write out a metric for a wormhole geometry and patch the two ends together in asymptotically flat spacetime where there is a large relative motion between the two ends. You will see that time dilation occurs, and that a "time lag" builds up between the two ends that allows for something like time travel. I don't expect most people here to actually do this (for one thing, it is rather tedious), but it is the most direct way of showing what is going on. I have tried to present intuitive arguments for why this occurs using observers near the wormholes, which I think is an accurate representation of what is happening but which is a lot easier to follow than the math of a bunch of curved spacetime coordinates.

We can say that matter warps space, or that matter grips space, and this can serve as a sort of post-hoc intuition once you know what is already going on, but it doesn't work for analysis. The useful statement is thatR_mu,nu - 0.5 g_mu,nu R = 8 pi T_mu,nuwhere T_mu,nu is the stress energy tensor (if which the mass contribution is usually the largest part, in the 0,0 position), R_mu,nu is the Ricci tensor (which describes spacetime curvature), g_mu,nu is the metric (another way of describing curved spacetime) and R is the scalar curvature. That's Einstein's equation, and it has been experimentally verified to high accuracy.

In general relativity, a wormhole geometry acts just like any other massive object when you are away from the exotic "matter" the props open the throat. The principle of relativity tells us that the physics are unaffected if it is moving or not, thus it cannot "drag" space along with it (otherwise you would know if it were moving, and you wound have an absolute frame of reference).

One principle which naturally falls out of general relativity is that if you can get far enough away from an area of curved spacetime that spacetime is approximately flat (the so called "asymptotically flat" region), then whatever happens inside the highly curved space-time region, the momentum (and energy) of that curved spacetime region as a whole is strictly conserved. Thus, if your ability to "grab space" is not infinite in extent (or at least large enough in extent that it extends to some other region of highly curved spacetime, like a black hole or pulsar), you cannot change your velocity by pulling on the space you grabbed. This limits reactionless drives to those where you can change your location but not change your velocity. (At this point it is perfectly legitimate to say "Err, what? A change in location is velocity, blockhead." The distinction, and it is fairly subtle, is that the object being reactionlessly propelled is not changing its velocity with respect to its local inertial frame of reference, with the practical effect that when you turn the reactionless drive off, you would be at the same velocity that you were before.)

If you want your science fiction to have "space moving along in the wake of large masses", or reactionless drives that work by "grabbing space" to accelerate you, you will need to work in a world where general relativity is not a complete macroscopic description of the world we live in. If this is plausible future of our current world, the new description will need to also explain the observed effects that were explained by general relativity (the precession of Mercury's orbit, deflection of starlight in proximity to the sun, energy loss of close orbiting pulsars, and the like). No one expects an author to actually come up with such a mathematically rigorous theory, of course, and your readers would probably not care to hear about it, but it will help for your back story.

Rick: Heh. I can see how it could sound that way. In this case, negative energy regions of space-time have a definite meaning. Energy is the ability to do work, so it takes work to make a negative energy region from a zero energy region, and you can do work by returning a negative energy region to zero energy. That the energy also has an effect on the curvature of space-time is a rather nifty bonus.

Thank you, Luke. You obviously know way more than I can grasp about relatively, but it helps to read your (relatively) simplified explanations anyway.

I do have a question about this*:

In general relativity, a wormhole geometry acts just like any other massive object when you are away from the exotic "matter" the props open the throat. The principle of relativity tells us that the physics are unaffected if it is moving or not, thus it cannot "drag" space along with it (otherwise you would know if it were moving, and you wound have an absolute frame of reference).

I think I've read about very massive objects dragging along spacetime as they rapidly rotate or otherwise move. At least, this caused something called "frame dragging," which I interpreted as meaning that surrounding spacetime was grabbed and dragged along. Or does this mean something else?

I will accept, "This is far beyond your understanding" as an answer if it involves complicated physics that cannot be explained through the use of homey analogies involving, say, arrows, apples, trains, puppies and cows, or requires actual math or, for that matter, mre than the simplest arithmetic.

*Actually, I have questions about a lot of things, but for most of them I cannot be any more articulate than "Huh?"

Stevo Darkly: You do get a sort of "swirling" effect around a rapidly rotating massive object, an effect that is commonly called frame dragging. This is allowed because a rotating frame of reference is not an inertial frame (only inertial frames are equivalent vy the principle of relativity). Another manifestation that is sometimes called frame dragging is the gravitational analogue of magnetism - a moving charged object creates a field that swirls around it with an axis along its direction of motion that deflects other charged particles, called a magnetic field (note, though, that this is frame dependant - what in one frame is a pure electric field becomes a mixed electric and magnetic field in another frame. Also note that the charges will be deflected toward or away from each other by the field, not around one another, because the deflection is perpendicular to the field and the charge's direction of motion). Similarly, if you move past a massive object at high relative velocity, you will be deflected a different amount than you would expect from its static gravitational field alone - an effect you could interperit as a "gravito-magnetic" field that swirls around the massive object in the direction of your motion. Note that since your deflection is perpendicular to the "gravito-magnetic" field and to your direction of motion, the "gravito-magnetic" field deflects you toward or away from the massive object, not around it. In the weak field approximation, where Newton's law of universal gravitation approximately holds, the "gravito-magnetic field" arises from the same effect that gives us a magnetic field from a static electric field (for those familar with Lorentz transforms, this is a Lorentz transform of the field tensor). For masses large enough that you can't treat gravity as a field, you still get these frame dragging effects - such as the Kerr metric for rotating black holes, but your can no longer treat it quite as a magnetic analogue.

Relativity allows for wormholes right? Well, how useful would they be? Would the ends track with planetary bodies or stars? Or would it be like a loose string not bound to anything?

If both ends are loose, and they aren't moving relative to each other, then passing through shouldn't have causality issues. But finding either end or having it be useful in any way would be questionable. Likewise, it would be unlikely to form from nothingness.

If one end is 'bound' to another object, but the other end is loose, then once again no causality, although the usefulness is minimal since the loose end could be moving at ludicrous speeds relative to the local environment.

If both ends are bound, then there is very likely a very large stress trying to tear it apart. I can't see it lasting for more than a few moments unless the two end points were synchronized and thus not moving relative to one another. This would be the case in short hops within the same star system.

Still, I'm not seeing how any wormhole would be useful in the stable transport sort of way.

Relativity basically does not forbid wormholes. Since a wormhole is a special case of black holes, and black holes exist, it is entirely possible for wormholes to exist, however they have a tendency to want to pinch off at the moment of creation, unless you act on them with dark matter/energy.

We know there is dark matter/energy because the Universe's outward expansion is accelerating, which it shouldn't be because of gravity. This mean that there's something out there creating anti-gravity, and there's more anti-gravity than gravity in the Universe. Unfortunately this dark matter is sufficiently exotic that it does not interact in any electromagnetic way with other matter, only gravitationally. Since we haven't found a graviton yet, we don't know how to find it on a quantum level, we only infer its existence at the astrophysical level.

(Quantum mechanics and astrophysics have so far declined to agree with each other, which is sort of an issue we're working on)

So technically it should be possible to maintain a wormhole open if you "coat" the inside of it with this anti-gravitational matter.

As far as "stretching" the wormhole as the two ends are moving with respect to each other, this impression is only true in the normal 3-dimensional Universe you are thinking about. If you move two ends of a string, either the string is elastic (stretching the rubber band) or it is not (if it is plastic you either break it or dislodge it)

Neither of these actually hold true in the weird multidimensional mathematical models of either Superstring Theory (11 dimension) or the Brain theory (which involves a parallel dimension closer to us than the size of an atom) In either of these models the spatial "distance" between the two "endpoint" of the wormhole do not apply. The two ends of the wormhole as a space-time causal link, not a physical spatial link.

I'm not sure my explanation is any clearer. I don't have Luke's talent is actually explaining or even understanding it. I think I am hearing a lady singing in Welsh at the back of my brain, but I could just be going crazy trying to wrap my brain around it.

And hey, Stephen Hawking may be wrong about it, and I certainly don't understand this stuff anywhere near as much as he does. O.o

Citizen Joe: The ends of wormholes follow the same paths that any object would. They have mass, and if you exert a force on them they accelerate in accordance with Newton's second law. If you have one in a star system, it will follow a Keplerian orbit around that star just as would any bit of inert matter. If you keep your wormhole on a planet, you will need to support it against gravity (perhaps just resting on the ground will do this, we do not know). Each end moves independently on its own trajectory, irregardless of what the other end is doing. The main complication is that a wormhole absorbs the momentum as well as the mass of anything going through, and gives up the momentum as well as mass of anything coming out. Thus, traffic through a wormhole will generate forces that can alter its trajectory.

All of the wormhole geometries I am familiar with don't have the ends moving with respect to each other through the wormhole, as much as they might move with respect to each other through flat space-time. That is, look through the wormhole and the other end is a constant distance away, always. Look at the other end through flat space-time through a telescope and you might see the other end moving quite a bit.

You can see how a wormhole is useful for travel by considering our previous example - one end on Earth and one on Terra Nova. I am on Earth and I want to visit Terra Nova. I step into the wormhole end on Earth, jump across the wormhole tunnel (we'll make this one have a short tunnel, just because we want to, but you can have a long tunnel, or just a vanishingly thin portal if you prefer), and you will be on Terra Nova, 100 light years away. When you get bored of life on the frontier, you can go back to the wormhole, jump through, and be back on Earth. So long as the wormhole does not take you further backward or forward in time than 100 years, it is impossible to violate causality (we say that they have a space-like separation). So long as the separation is space-like, it is thought that the wormhole mouths exert no forces on each other, and the wormhole is stable.

However, what happens if Terra Nova orbits a heavier star than Earth, so it is orbiting faster and deeper in a gravity well. It is also farther into the galaxy's gravity well. Uh oh! The Terra Nova end of the wormhole is continuing to experience extra time dilation not felt by the Earth end. Eventually, more than 100 years of time lag will build up. Perhaps Terra Nova's sun (and thus Terra Nova itself) is drifting toward Earth, so the distance is getting closer. As soon as the time lag (in years) is more than the distance (in light years), you can use the wormhole to go back in time and then send a lasercom signal to yourself before you left. (Terminology: when the time lag is exactly equal to the distance, we say the separation is light-like. When the time lag is more than the distance, we say the separation is space-like.) It is thought that as soon as you get a light-like separation, the path back in time through the wormhole and then returning through flat-space forms a perfect amplifier for radio, light, and any other electromagnetic signal (not to mention gravitational waves). Fluctuations in these waves spontaneously appear and build up to such huge amplitudes that they either destroy your wormhole or exert a force that pushes the wormhole ends apart so as to keep them from forming a time machine.

Fortunately, there is a way to prevent this. Charge up your wormhole, shrink it back down to what it was when it was traveling, and put the Earth end in a cyclotron. Spin it up to ultrarelativistic speeds. The time dilation on the Earth end decreases your time lag across the wormhole. Stop spinning the earth end when the time lag gets small enough, discharge the wormhole, inflate it back up to usable dimensions again, and open it back up for travelers.

Putting wormhole mouths on the surface of worlds seems like there would always be a huge conservation of momentum issue. The worlds would have to be the same diameter and spin at the same rate with parallel axes. Both would have to orbit their stars at the same speed, distance and parallel angles. Both stars would have to be the same mass. Both stars would have to be traveling at the same rate on a parallel course. A space based wormhole would be more feasible since it only requires synchronized stars. While you claim all this time dilation effects, I suspect that the momentum differentials would just shred the wormhole. Perhaps is also means that stable wormholes would only form out in 'flat space' where large gravitic bodies aren't creating undue stresses on the mouths.

Citizen Joe: There is no conservation of momentum issue. Momentum is automatically conserved locally. Here's an example:

Suppose we have a stationary wormhole mouth with mass M. It has a maglev train track going through it. A maglev trolley with mass m and velocity v floats along the track and through the wormhole. Before the trolley goes through, the total momentum of the system is M * 0 + m * v = m * v. After the trolley goes through, the wormhole mouth has a mass of M + m and a velocity of v * m / (M + m),drifting along the track.The total momentum of the system is (M + m) * v * m / (M + m) = m * v,the same as before. Momentum and mass (energy, actually, and also angular momentum and electric charge) are conserved locally, with no reference at all to what is going on at the other end. (In practice, the wormhole end will probably be braced if it is on a planet's surface, not free floating along the track. In this case the wormhole exerts a force on the braces, which in turn push back on the wormhole via Newton's third law of motion. This transfers the momentum between the planet and the wormhole as the trolley goes through which keeps the wormhole stationary with respect to the planet).

But let's look at the other end for a moment. This end has a mouth with a mass M', also initially at rest. The initial momentum of the system is M' * 0 = 0.When the trolley comes out of the mouth at velocity v, the mass of the mouth decreases to M' - mand it acquires a velocity of- v * m / (M' -m)backwards along the track such that the total momentum is still[m * v] + [(M' - m) * (- v * m / (M' - m))] = 0.Again, momentum and mass are conserved locally. There is no dependence on the dynamics of the other end of the wormhole.

However, now we have an interesting question. What if the mass of the trolley is larger than the mass of the wormhole mouth that the trolley comes out of? The conservation of mass tells us that the wormhole mouth ends up with a negative mass! Negative mass is weird - if you push on it, it comes toward you! It seems unphysical. Perhaps it is - some relations in quantum mechanics indicate that regions with negative energy (mass) density must be bounded with regions of positive energy (mass) density and with more positive energy (mass) than negative energy (mass). If this holds, a wormhole will never acquire negative mass. Perhaps it collapses before this can happen (shearing off anything inside of it that is about to give one end negative mass). Perhaps some sort of force develops which bounces back anything in it that is about to give one end negative mass. Or maybe you really can have negative mass general relativistic (as opposed to quantum mechanical) objects. We do not know.

Personally, I think it is more interesting if you have to keep the mass of both ends positive. Now you need to be careful to balance the mass going through, which adds an interesting and novel constraint on our wormholes that is not generally seen in FTL used in fiction. But my preference is not certain, you can write stories with negative mass wormholes in them and still have them be hard science fiction if that is what you prefer.

Jean Remy: I thought you are French? Anyone who speaks English as a second language as well as you do has nothing to be ashamed of even though you don't use words like "irregardless". I wouldn't be able to tell you were a native French speaker by your grammar, only by the context of your comments, Your English is certainly much better than my French, which despite six years of schooling in your beautiful language I can still barely use.

Try this. Disjoin mass as a property of matter and make it the distortion effect caused by matter. I.e. the mass of an object is the curvature it places on space time. Now when you send an object through a wormhole, it doesn't affect the mouth but rather distorts space around the mouth of the wormhole in such a way that local momentum is conserved. This would likely take the form of a ripple in space time that propagates out until it can be absorbed by actual matter. Imagine traveling along in your ship when you suddenly encounter a distortion wave that imbues your vessel with the lost momentum. Polite people would probably form the wormhole with some sort of gate ring. This would provide the power generation and machinery to form the wormhole, but it would also serve to ground out the momentum wave. Vessels capable of forming their own wormholes would leave a deadly space time wake as they depart. On the other end, a similar wake would occur. This time space warping is taking the place of the negative mass.

Citizen Joe: An interesting speculation, but it does not work within the confines of general relativity. For one thing, a distant observer would see a change in the mass of the vicinity of the wormhole, from the mass of the mouth plus an object, to just the mass of the mouth. The distortion in gravity would manifest as a propagating wave of "less mass", causing a lower gravitational acceleration toward the vicinity of the wormhole. In particular, a physicist would classify this as a longitudinal (field changes in the direction of propagation), monopole (field changes the same amount in all directions) gravitational wave. However, it is known that longitudinal waves cannot propagate through space-time - the only gravitational waves that can propagate are those which have a field perpendicular to the direction of propagation, which stretch space-time in one perpendicular direction while squeezing in the other perpendicular direction. (In addition, all gravitational waves are quadrupole waves, having at least four lobes where the radiation is most intense with "nodes" between them of zero intensity).

You can go further, and prove, quite generally, that in general relativity if you take a closed space-like surface in which the space-time at the surface is nearly flat (the asymptotically flat region) but which encloses regions which may have significant space-time curvature, then the change of the total mass and momentum within the surface is equal to the amount that flows in through the surface minus the amount that flows out through the surface. So, for example, if we take an imaginary sphere that surrounds our wormhole mouth out at a large enough distance that the gravity of the wormhole does not appreciably distort the sphere, then if nothing goes through the sphere the mass inside the sphere cannot change. Thus, if something comes out of the wormhole mouth, the mass of the wormhole mouth plus the mass of the thing that came out of it must equal the mass of the wormhole mouth before the thing came out of it.

If you want to have propagating mass distortions like you are describing, you have to assume that general relativity is not an accurate description of the behavior of space-time.

"Jean Remy: I thought you are French? Anyone who speaks English as a second language as well as you do has nothing to be ashamed of"

I've had an interesting scholastic career. But yes, in most practically appreciable respect I am absolutely bilingual. I do not "translate" my thoughts from french to english, I think in french or in english depending on which language I have to use. I am writing several novels in english and I wrote one in french. Honestly there is no appreciable difference in "difficulty" in either language.

Citizen Joe: Re-reading your post, I think I may have misinterpreted it. You are saying that when an object goes through a wormhole, the wormhole mass stays the same, but a "mass wave" ripples out. When an object comes out of a wormhole, you also get a mass wave, but one with negative mass. The former is not entirely implausible - you might imagine electromagnetic fields whose total energy equaled the mass-energy of the thing that went through the wormhole being emitted as a wave (giving you the equivalent of a million Hiroshima bombs in radio waves for a mid-sized spacecraft). It is hard to think of something that could give you the latter kind of wave, the kind with negative energy. One possibility are so called "advanced waves". These violate causality, but who knows. Advanced waves are essentially incoming waves that converge to a point at the time the source creates the waves. This means of course, that the waves were in existence before their creation, marching inward to their source. This is not entirely unphysical - you can create advanced waves with a converging lens, the physics is the same and the absorption of the converging waves corresponds to the creation of advanced waves. However, assuming that every time an object comes out of a wormhole there just happens to be incoming waves created far back in the hazy early universe that exactly cancel the mass change caused by the object coming out requires that the second law of thermodynamics be broken in a big way.

General Relativity requires 'Dark Matter' or 'Negative Mass' or 'Angels on Pinhead' to explain some of the anomalies out there. I have no problem adding 'Detached Mass Momentum Propagation Wave' to the pile. As a story teller, I especially have no problem as it makes wormhole travel difficult/dangerous/interesting and limits its misuse for turning it into a weapon.

Citizen Joe: It seems that this mass/momentum wave of yours would make it simple to turn your wormhole into a weapon. Essentially, you would have a re-usable nuke with a yield of the mass of whatever you can stuff into it times c^2 (22,500 megatons per ton of matter shoved through). On the other side - well, you get a wave with negative energy density. Presumably it will suck the energy out of things it interacts with, until it has absorbed 22,500 megatons of energy per ton of stuff that came out. This could, for example, take the form of destroying the matter it encounters.

It isn't weaponizable because you can't really direct the target. Might be closer to a nuclear minefield. Anyway, that is why 'polite' people put these things near momentum sinks, gas giants, stars etc. It can be used as a weapon, but only by shear luck or extreme planning.

So if a vessel leaves system A and enters system B, it creates four distortion waves. First, a positive wave propagates out in system A with the same momentum as the departing ship. This would be a relatively small wave. Next, a negative wave would be formed at system B which balances out the momentum imparted by the incoming ship. Then the recoil waves occur, feeding back through the wormhole. System A gets a negative wave which balances the ship's mass times the relative velocity of system B to A while system B gets a negative wave equal to the mass times the relative velocity of system A to B. Properly timed and synchronized passages could minimize these recoil waves which adds another layer of interest to wormhole travel. Likewise, superior piloting could use the primary wave to cancel out the recoil wave. But as they say in Mass Effect II, "You do not EYEBALL it!"

Citizen Joe: In order to conserve both energy and momentum, the "momentum wave" after a spacecraft enters the wormhole must also have energy equal to the mass-energy + kinetic energy of the spacecraft. It will be slightly unbalanced, with more in the forward direction, in order to conserve momentum. Maybe it is just me, but this doesn't seem to be a relatively small wave, not for macroscopically sized spacecraft.

I am not even sure where those "waves" come from at all. Conservation of, let's call it Information, works at a universal level. If the wormhole were to be causally disconnected from the Universe then yes, the removal of this ship from that Universe would cause a burst of energy equivalent to the ship's potential (good ol E=mc^2) However the wormhole is not causally disconnected from the Universe since causality is maintained throughout, otherwise it cannot exist. Momentum is conserved because the ship is spat out on the other side of the wormhole and Conservation of Information is respected in the wormhole (as much information enters the wormhole as exits)

Otherwise, if energy enters the system of the Universe when the ship enters the Wormhole, then information has to be destroyed when the ship comes back into the Universe (at the other end or the Wormhole. In other words, the ship enters the causally disconnected wormhole and leaves behind a wave of energy equivalent to its energy potential (as stated above) All well and good as Ordered Information becomes Disordered information. However, when the ship re-enters the Universe, the same amount of energy has to leave it. Now disordered information is replaced suddenly by ordered information, violating entropy as well. Not only that but you could then say the energy wave enters the wormhole *before* it exits, shooting down causality.

Either wormholes are possible, and there are no resulting waves, or it is simply impossible to create, maintain, or send information through them, because otherwise you'd be violating causality *and* thermodynamics in an attempt to not violate conservation of momentum.

I think. This is stuff the greatest physicists in the world are trying to piece together and I am not one of them, so it is probably better for my sanity to give up while I'm ahead...

Feel free to disregard the above. I think if I try to read it again I'll go crazy.

I'm not sure where the waves come from either. I can kinda see where you might get the emission of "something" to balance mass and momentum at the mouth that you go in to, but negative energy waves coming out of the exit mouth look like they sink the concept.

But needless to say I enjoyed the mental image of a trolley emerging from a wormhole ...

Slightly mets, it is rather appropriate that both of the digressive discussions in this thread, on communications and FTL, come in a post about torchships, which more or less define the limit of traditional, demi plausible spacecraft.

Not to mention that both communications and FTL featured prominently in Heinlein stories involving torchships.

On 'irregardless,' there's an argument that however much it grates on some of our ears, usage has made it standard. See Language Log for frequent discussion of the inherently fuzzy nature of correctitude in language.

If I understand the description of wormholes correctly, once you sent a wormhole from Earth to Terra Nova, you could not send back a different wormhole from Terra NOva to Earth without destroying one of them ( if you did, you could use the Earth-TerraNova-Earth bridge to go 200 years in Earth future and return with precious informations about who won the World Cup of 2051...).In fact, once you opened a wormhole route to a destination, you could not send a new wormhole from that destination anywhere inside the light-cone of the original source point.What kind of effect would take care of so conveniently saving causality?

A previous commenter, ElAntonius, said that: "which is also convenient from a science fiction perspective because if we can put the wormhole on earth, there ceases to be a need for manned space travel."

While it may be true that it'll eliminate most of the 'need' for manned spaceflight I would still see it as a relatively common thing because with wormholes that you can go through physically you've eliminated the single greatest hurdle to getting into space: Getting off the damn planet to begin with.

After all, what's stopping you from parking one end of the wormhole in orbit and the other on the surface in a giant vacuum chamber that you enter through an airlock?

Now all of the sudden ideas like your well-to-do crazy uncle Ernie wanting to go off to an asteroid so he and his 20 cats can form the free-colony of Ernietopia become possible. (If still not the best of ideas.) His super-heavy, home-made, welded together from plate steel ship can be put into orbit along with all of the reaction mass it'll need with relative ease because you're no longer having to use expensive chemical rockets/laser launchers/whatever to put it up there to begin with.

Welcome! I'm trying to keep my mouth mostly quiet as most of this discussion has been way over my head. Brane omelet, omnomnom

Anyway, if we put a wormhole on earth and one in orbit, then sure we have a REALLY cheap earth-to-orbit mechanism. But the problem is that there's no reason to travel anywhere...if the asteroids interest us, why not just put the other end of the wormhole near where we want to be? Send out an unmanned spaceship to move it around if you want, but why put people on the end of it?

It's not that manned vehicles in space won't exist, we still might need some LEO ferries and that sort of thing, but in any scenario where we could travel by rocket, using an unammed drone to place a wormhole where we want it and then stepping through in a puddle jumper is cheaper.

That's why most science fiction stories that feature wormhole-like FTL place them out somewhere really inconvenient, well beyond LEO. It lets them write stories that are basically traditional spaceship stories.

Speaking of which, the most popular locations for wormholes seem to be lagrange points or right around the Pluto zone. I find that interesting: it's an appropriate pioneering feel to having a tiny little outpost be the last spot of civilization before you continue out west.

I always wondered...what if we instead put our FTL jump point near the sun...say, right around Mercury.

Does it strike anyone else that that would have interesting implications?

Jean, when you mentioned 'brain theory' a bit upthread, I think you meant 'brane theory.' Given the head-exploding aspect of all this, an understandable typo!

The setting for Attack Vector, from Ad Astra Games, has FTL jump points that are more or less at Mercury distance, which certainly poses some normal-space design challenges for the ships, i.e. not baking en route to the jump point.

On purely aesthetic grounds I grump at this twist - starships should head 'outward,' toward the stars. Which is another way of saying that our general tendency with FTL ships is to rig things to get Traditional Spaceship Stories.

Oddly enough, with ground based stargate FTL it seems equally appropriate that you should go into a tunnel to make the jump, rather than a mere ground level facility.

Francesco: Exactly right. Well, not quite inside the future light con - if you send the wormholes slowly so that they only built up a time lag of, say, 6 months, you could have a wormhole from Earth to Terra Nova, and another from Terra Nova to anywhere further than a light year of Earth.

There is a way around this. I mentioned taking the Earth end of the wormhole, putting it in a particle accelerator, and letting it go around in circles at ultrarelativistic speeds to reduce the time lag. If you do this for long enough, you can completely get rid of the time lag, or even reverse it. For the Earth - Terra Nova wormhole, it will require the wormhole to go around and around in the accelerator for at least 100 years, although you could always stop it every so often to let people and equipment through. Note that on Terra Nova it will seem to be much less than 100 years, since the wormhole end on earth is undergoing time dilation. This trick would allow you to build round trip wormhole networks, but you will need to be careful to keep them all synchronized to prevent time machines.

Also, the powers on Earth might not want this. Suppose we Earthlings send a wormhole to Tera Nova. And then we send another to New Carolina, 100 light years away in another direction. And maybe other wormholes to Homestead, and Johnsworld, and Zemynia, and perhaps a few other colonies. In order to trade with each other, these colonies must route their traffic through Earth, since they cannot send wormholes to each other without making a time machine. The colonies can extend their wormhole networks away from earth, but you end up with a branching tree-like network in which Earth is at the nexus, the root node, and thus all trade between major branches will come through Earth. You can see how there would be those on Earth who would be making a lot of money off of this.

Nick P. : One minor detail - remember that mass must be conserved locally (well, energy must be conserved locally, but to our approximation it would be mass). So if uncle Ernie wants to put his super-heavy home made ship into orbit (and assuming net negative masses are impossible), he will need to find an equal mass of stuff in orbit to bring back. The sequence might go something like this:(1) Ernie launches a 10 nanogram wormhole mouth up into orbit. The corresponding mouth stays at home with him (also 10 nanograms).(2) The orbiting wormhole mouth finds a 1,000,000 ton asteroid up there, and "eats" it. The asteroid is now inside the wormhole. The orbiting wormhole mouth now has a mass of 1,000,000 tons (plus ten nanograms, but I'll ignore that for now).(3) Uncle Ernie puts his 400,000 ton Ernietopia habitat through the wormhole. The wormhole end back at home has a mass of 400,000 tons and the orbiting end has a mass of 600,000 tons.(4) Ernie still has 1,000,000 tons of stuff inside his wormhole.

It's this minor detail that makes getting to empty space difficult, but it certainly makes getting to other planets easier.

Jim Baerg: That is certainly an option, but it must be done while one end of the wormhole is en route. For example, let's consider the case of Earth, Terra Nova, and Johnsworld, all 100 light years from each other. Earth already has a wormhole to Terra Nova and Johnsworld, with a 99 year, 11 month lime lag (that is, going from Earth to Terra Nova or Johnsworld takes you 99 years, 11 months into the future, while going from Terra Nova or Johnsworld to Earth takes you 99 years, 11 months back in time). Now the merchants at Terra Nova want to cut out the middleman and get around the crippling tariffs at Earth, so they plan to establish a wormhole route to Johnsworld. They launch a wormhole mouth at the time dilation factor of 1200 we've been using before. As soon as the wormhole mouth gets roughly half-way there, light can go from Terra Nova back in time 99 years 11 months to Earth, forward in time 99 years 11 months to Johnsworld, spend 50 light years going through flat-space from Johnsworld to the wormhole, and then go back in time 50 years to Terra Nova to arrive as soon as it left. At this point, the weakest link the the wormhole network breaks to prevent a time machine from occurring - probably the Tera Nova to Johnsworld wormhole since it is still small and weak and not reinforced like the wormholes to earth are. Thus, the merchants at Terra Nova have to put their end in a cyclotron and spin it around at a time dilation factor of 1200. Now both wormhole ends have the same time dilation, and no time lag builds up. The major drawback is that the Terra Nova merchants have to wait 100 years until they can trade with Johnsworld, rather than 1 month if they can let time dilation do its thing.

Now, if you can propel a wormhole that is strong and heavily reinforced, even though it is low mass for travel, then you can get wormhole wars! The Terra Nova merchants can try to bully through with their extra strong wormhole to Johnsworld, hoping one of the Earth wormholes breaks first. Probably, they want the Johnsworld-Earth wormhole to break, leaving them at a trade nexus. At this point, everyone starts beefing up their wormholes so that theirs is not the weak link in the chain.

But if you have an equilateral triangle like you suggest (apparently 100 light years to a side) then it's not a problem because it is at the limit of the light-cone, no? The distance between Johnsworld and Terra Nova has to be less than between Either TN or JW's from Earth...

I think I am completely lost now actually.

Hrm just had an idea but my brain hurts to much to figure it out. What if the wormhole network instead of launching a wormhole from Earth to TN would involve sending wormhole from both TN and Earth at the same time and have them meet in the middle? Each wormhole travels then 50 ly's for half a month, and a ship using one half of the tunnel will be propelled 50 years in the future, then drop back fifty years in the past when arriving at TN, essentially a near-instantaneous trip if we consider TN and Earth a closed system on an asymptotically flat space where special relativity applies.

My brain just fried. I need to step away from this. And I thought quantum mechanics were mind-bending.

Jean Remy: It's not so much a light cone (which after all, is defined for an event, and if you wait long enough for any event on Terra Nova, say, the world line of Earth and of Johnsworld will eventually enter its light cone). It is that if you are on Terra Nova and you go to Earth (going back 99 years, 11 months) and then go to Johnsworld (going forward 99 years, 11 months), you end up at the same time coordinate as you left. Now, to get back to your original position, you need to spend at least 100 years crossing that 100 light years between Johnsworld and Terra Nova. You can't possibly get back before you left, and you are perfectly safe. The same argument works no matter how far apart Terra Nova and Johnsworld are, so long as the same amount of time is lost and gained on the trip to Earth.

Just to shake things up, suppose the people on Earth decide to put the Johnsworld wormhole in the cyclotron and let it sit there for a bit over 100 years, such that going from earth to Johnsworld takes you back in time by one month. Now things are dangerous - from Terra Nova to earth you go back in time by 99 years, 11 months, from Earth to Johnsworld you go back by another month, taking you back by a total time coordinate of 100 years. Since it takes 100 years for light to get from Johnsworld to Terra Nova, you can send a signal to yourself that arrives just as soon as you leave - you are on the verge of making a time machine, and this is the condition for the perfect resonator that is thought to blow up any wormhole network where it occurs (so far it has only been shown for single wormholes and two wormhole networks, but is suspected to work for any number of wormholes).

I find it useful to make a simple diagram of the paths - say put three dots as vertices of a triangle and label the vertices as Earth, Terra Nova, and Johnsworld. Now you can draw in the change in time coordinate for travel along each of the legs of the triangle (note that this depends on which way you go, if there is a wormhole on that path). If any round trip journey can get you back before you left, you have a time machine and your wormhole network would be destroyed before it could be made (according to our best guess of how wormholes work).

Our problem is that we don't have a general sense of how to understand time in this context - I mean in human terms.

It is teasing around in the edge of my mind, that places are uptime and downtime with respect to each other.* So long as you know which, ordinary people don't have to work out GR to figure out hotel reservations, or even do long term strategic planning like sending out wormhole links.

I think there is a better way for 2 colonies to connect without going through the Sol system. They both send a worm hole to a system that is farther from sol than they are. It shouldn't be too hard to pick the position timing & time dilation so that the resulting wormhole paths don't create a time machine.

@Luke, I've understood (at least in a general view) the constraints under wich it would be possible to have wormhole FTL without breaking cusality. Only I've not understood if there're some specific mathematical pointers, in general relativity or quantum mechanics, to the mechanism that would make wormholes instatly collapse before being able to violate even for a brief moment causality, or if we're simply strongly hoping that there's such a mechanism, for the sake of causality (and because it would make a little more likely FTL).I surely hope we'll not have to invoke something like the Escathon of Charles Stross's Singularity Sky and Iron Sunrise, to protect causality... :-P

Jim Baerg: I think that just might work. You would need to be very careful to closely synchronize the wormholes from both ends.

Franceco: Kip Thorne, Sung-Won Kim, and Stephen Hawking have shown that for a single wormhole, electromagnetic vacuum fluctuations will be focused to a strong pulse just at the moment the wormhole becomes a time machine. They concluded that this may be intense enough to destroy the wormhole (Kim and Thorne, Physical Review D, vol 43, article 3939, 1991). Hawking strongly believes this to be the case, and advanced the chronology protection postulate that vacuum fluctuations will always destroy a time machine before it forms. Thorne suspects he is right. Matt Visser later showed that a single wormhole will almost certainly be either destroyed or subject to such intense forces that it will "bounce away" before becoming a time machine, and that any configuration of two wormholes will probably also not be able to become a time machine (Visser, Lorentzian Wormholes: From Einstein to Hawking, AIP press, 1996). As far as I am aware, there is still no general proof that any configuration of wormholes will be prevented by vacuum fluctuations from becoming a time machine.

Von: the French one was an adventure novel, think Indiana Jones, and is so embarrassing that it has been shelved into a deep dark drawer and should never see the light of day. But it was a first effort and did teach me a lot about writing (mostly how not to do it)

The other novels are SF and form a series. They are currently under revision process. The first one was rejected by quite a few agents, however I received some three-page long rejections with a lot of advice, positive comments and invitations to resubmit after revision.

In fact I would like to thank Rick (once again) and all the commenters on this blog, and Atomic Rockets. They have been very useful in that process and have helped me flesh out my world-building. As far as samples, perhaps at some point.

Three page rejection letters are a very good sign! There is a horrible, cautionary (and thus probably apocryphal) story about a writer who got a 10 page single spaced letter from an editor, didn't understand the implication, and was so crushed they stuck the manuscript in a bottom drawer and gave up the whole idea.

Thanks. Yeah I am fully aware of the implications of a three page rejection letter. I don't keep the thin slip of papers with the formulaic one sentence "not interested at this time" ones, but the three page ones I surely keep.

I really hope that cautionary tale is a legend!

Although another cautionary tale: one of those agents that seemed helpful tried to pawn me off to a "book doctor"... a practice which is deemed unethical by the Association of Author Representatives.

Advice to any other would-be writers: research is not just for your book. I did almost as much research into how to sell my books as I did writing them. Also, a thick skin helps. And wear sunscreen.

Good luck on your literary ambitions, let us know if/when you are published.

WRT torch ships, this blog post has an interesting table of time to various planetary targets using this formulation:

"Assuming a 1,000 ton space vehicle and constant acceleration at 0.722 m/s2, which an exhaust velocity of 10,000 km/s with a mass-flow rate equal to “Daedalus’s” second-stage would produce, we get the following figures for travel times to various Solar System destinations."

Those performance stats are certainly torchlike, and in fact an exhaust velocity of 10,000 km/s is wasteful for nearly all Solar System travel - on most routes you just don't have time to reach more than a few hundred km/s.

Using STL starship technology on interplanetary routes is like using a jet plane to get around town.

As for the jet argument, I would consider this more like intercity commuter service. For me, taking flight from Toronto to Ottawa is only 2 hours including time at the airport vs 7+ hr by car or train. The actual flight time is even less, so the ratio is heavily in favour of air travel.

Freight will certainly go by the equivalent of container ship or freight train, trading cost for time, but there will certainly be a class of traveler who isn't willing to sit on a cycler for years at a time, so I am sure a high speed transportation segment will arise.

I'll agree with Thucydides on this one, in saying there's no kill like overkill.

There's no such thing as going somewhere "too fast". At least in terms of military strategy you'll want the ability to get somewhere faster than anyone else can, and damn the price at the pump. It is more costly to arrive at a battle late (and for want of a horse)

I would also expect the richer people, should a CEO venture this far, will want to cut their journeys as short as possible, and will then pawn off the tremendous cost to their expense accounts.

I guess I don't understand your "gearing" analogy. I think transit times should be taken into consideration before any other concern.

I assume those numbers (mercury in 8 days) are for brachistochrone orbits. This ship puts pretty much the entire inner solar system at one to two weeks travel time, which allows for even perishables to be shipped. This puts us in the era of Titanic and QE2 and the Orient Express, with regular travel open to the upper middle class. The lower middle class can afford a one way trip in the lower decks to try their luck on the new colonies.

The great adage that a ship doesn't earn money while in transit will continue to hold.

If I am still missing the point, could you please be clearer? The problem with Earth-bound metaphors is that they really don't mean that much in space. You can throw gearboxes at me and I can throw steamships at you all day, but that just probably mean we'll miss each other because gearboxes and steamships are pretty hard to throw, especially steamships so I am at a disadvantage, and I broke another metaphor there.

Jean Remy: Consider a 1,000 ton spacecraft with a 10,000 km/s exhaust velocity and an acceleration of 0.722 m/s/s. For a 1 AU trip at constant acceleration, flipping at the midpoint, it will take 10.5 days and consume 66 tons of propellant/fuel.

Now let's add extra mass into the exhaust stream, so that the spacecraft uses propellant at 16 times the rate but expells it at 1/4 the exhaust velocity (thus keeping the same power). This brings the acceleration up to 2.89 m/s/s. We will accelerate for 1/10 the distance, drift for 8/10 the distance, and then decelerate for 1/10 the distance. The trip now takes 7 days and uses 240 tons of propellant, of which only 7 tons is fuel.

Bulk inert (non-fuel) propellant is probably cheap (water or hydrogen). Fuel is probably expensive (He-3 and D). The second option gets you there faster and cheaper.

In Rick's analogy, high exhaust velocity, low thrust, low propellant flow corresponds to high gear. Low exhaust velocity, high thrust, high propellant flow is low gear. In this case, a lower gear than the default "interstellar" Daedelus thrust parameters is preferable.

A T3 line only has about 44 Mb/s capacity. So I'm going to err on the side of skepticism on the Terabit capacities."

A T3 line is primitive tech by modern standards, co-ax wires from the 1950s. Fiber optic is at least 10s of Gb/s commercial, and 10-100 Tb/s in the lab.

Interplanetary comms rely heavily on error-correcting codes, not just error-detecting and resend. You can create data packets such that expected noise patterns don't degrade your ability to reconstruct the packet.

More on the cost of e-mail.A terabit/s channel is over 100 gigabytes, or 1e11 bytes, per second.

A roughly average book is 100,000 words, or a megabyte. So the channel can transmit 100,000 books/s. Before using compression.

http://en.wikipedia.org/wiki/Wikipedia:Size_comparisons says English Wikipedia has over a billion words, 25x Britannica, about 10 gigabytes. Might not include revision history. The Yongle Encyclopedia was 370 million Chinese characters, under a gigabyte.

So the channel could probably transmit the text+HTML content of Wikipedia in a second.

British Museum has 150 million items, Library of Congress 120 million. Assuming 10 megabytes per item, that's ~ 2e8*1e7 = 2e15 bytes, so would take 20,000 seconds, or 6 hours, to transmit.

No compression, again. No pictures, either.

A top typist is about 100 words per minute, vs. my 30. Say 1000 bytes per 60 seconds, or 20 bytes per second. Assume a stable Earth population of 10 billion consists of top typists, all typing at speed 24/7, and everything they type is worth transmitting to Mars. 1e10 * 20 = 2e11 bytes/s, so we'd need two terabit channels to handle all the traffic.

More realistically, a single terabit channel should make interplanetary text e-mails writable by a human (no picture attachments) free, or at least very cheap.

Pictures, voice, and video are another matter. Still, with initial loadups and use of daily diffs, each planetary colony can probably keep local caches of the entire trans-Solar Internet, and certainly its textual content. Synchronizing edits of a system-wide Wiki would be 'interesting', though.

For that matter... from the world of RPG books, I know that a crude scan of something with a fair bit of color art can be around 100 MB in size. Call it 200. The British Museum has 150 million items, many dull print books, but never mind. Say 200 million items at 200 MB each, that's 4e16 bytes -- or 100,000 seconds of a 100 GByte/s channel, or 5 days.

The logical time to send a photographic archive of major libraries is probably[1] on the colony ship, or a later supply ship, but if you forget, it's probably faster and cheaper to beam it than to have a dedicated physical mail run.

[1] One could do the calculation of the marginal energy needed for some rocket-shipped extra hard drives, vs. the energy of transmission, but this one is not me tonight.

There's a simple solution to the eavesdropping problem. Park a relay a couple light-seconds out, and have the data beamed to it. It's then close enough to hit you with a pinpoint beam. As for torchships, I've done quite a bit of work on a small one, and am definitely in favor of gearing. It's got a 335 GW He3 torch cut by about a factor of 8 for cruise mode, and a factor of 217 for liftoff. This gives decent thrust, and on Luna, LOX is cheaper than D-He3.

'Gearing' is highly desirable even if the drive won't produce surface lift thrust from any significant body. Each deep space mission also has its own optimum balance of acceleration and delta v, favoring an adjustable drive.

This is why I instantly fell in love with the game Killzone 2. The good guy's space ships are Torchships, they arrive at the enemy planet within 2 weeks (this game takes place in the Alpha Centauri system). Infact, their engines are strong enough that the ships can actually use them to hover in the air, providing close fire support for the troops. Of course engines this powerful can also be used as weapons themselves, and infact they are. During a level one of the ships that remained in orbit uses its engine to destroy a large artillery gun on the surface. One very badass game!

I am intrigued by Luke's explanation of empire time. I have been led to believe that FTL is absolutely forbidden owing to the possibility of causality violation, but here it seems is a way to do FTL that is not forbidden, although it is technologically very difficult. So can Luke explain how Alcubierre's warp drive would avoid creating paradoxes? Given that the ship is at rest relative to the surrounding space inside the warp bubble,you could still send a message that would outrace a ray of light outside the bubble, creating the possibility of paradoxes. Or have I misunderstood it?

He would have to answer that question from theoretical perspective, not me, since it is way above my math pay grade. I'm not sure that the Alcubierre concept fits with his discussion at all. But my overall impression is that you can indeed have FTL, so long as you are careful to mind your temporal p's and q's.